Why do American jurisdictions generally prohibit ex post facto laws as opposed to European countries which apply lex mitior ("the milder law")?

Question

According to Wikipedia article related to Ex post facto law:

While American jurisdictions generally prohibit ex post facto laws, European countries apply the principle of lex mitior ("the milder law"). It provides that, if the law has changed after an offense was committed, the version of the law that applies is the one that is more advantageous for the accused. This means that ex post facto laws apply in European jurisdictions to the extent that they are the milder law.

I am curious about why this difference exists.

Question: Why do American jurisdictions generally prohibit ex post facto laws as opposed to European countries which apply lex mitior ("the milder law")?

Answer 1

Presumably because they seem to think that appeal is the only proper venue to fix such sentencing problems:

Recent initiatives concerning cannabis legalization, along with the European Union’s authoritative ruling [this is actually incorrectly referring to an ECHR ruling, which has even broader applicability], however, have brought the doctrine of lex mitior to The United States and Canada and sparked unresolved debates. While there are a variety of concerns, the most universal seems to be the legal grey area lex mitior creates and what this means for both the legal system’s integrity and its function. In accordance to most legal systems, cases are only retrospectively addressed through an appeal, for which both temporal and structural regulations exist. Appeals are a means of correcting a mistake that has allegedly compromised a defendant’s rights. In retroactive applications of a law, cases, which were theoretically conducted through proper channels at the time, are reopened and adjusted based on new information, which contradicts the very structure of a trial that determines a conviction based on the standard of beyond a reasonable doubt.

While retroactive laws raise various legal and ethical concerns, they may also alleviate the stress faced by the current judicial system in the long run and serve as an important step in effective criminal justice reform in some cases.

In the fairly recent (2012) and fairly controversial (5-4 decided)

Dorsey v. United States, the Supreme Court made clear that Congress possesses sole responsibility for drawing the line between justice and finality in sentencing. That is, when Congress passes a law reducing sentences, it must also choose whether to apply those reductions retroactively or to accept the sentence disparities between pre- and post-reform offenders that will otherwise inevitably result.

So basically, the US Supreme Court (unlike the EU and then CoE courts) found that such a (lex mitior) principle cannot be found in US jurisprudence, so Congress would have to legislate it explicitly, if it wants it. Subsequently Congress has done so for a limited purpose in the 2018 First Step Act:

The act, among many provisions, retroactively applies the Fair Sentencing Act [...]

Of note however as to its application:

The biggest immediate impact of the bill would be felt by nearly 2,600 federal prisoners convicted of crack offenses before 2010. That’s the year Congress, in the so-called Fair Sentencing Act, reduced the huge disparity in punishment between crack cocaine and the powdered form of the drug. The First Step Act would make the reform retroactive. Those eligible would still have to petition for release and go before a judge in a process that also involves input from prosecutors.

As noted in a 2012 review

Federal law prevents retroactive application of a law unless specifically mandated by the legislature. [citing: 1 U.S.C.A. § 109. The repeal of any statute shall not have the effect to release or extinguish any penalty, forfeiture, or liability incurred under such statute, unless the repealing Act shall so expressly provide, and such statute shall be treated as still remaining in force for the purpose of sustaining any proper action or prosecution for the enforcement of such penalty, forfeiture, or liability. The expiration of a temporary statute shall not have the effect to release or extinguish any penalty, forfeiture, or liability incurred under such statute, unless the temporary statute shall so expressly provide, and such statute shall be treated as still remaining in force for the purpose of sustaining any proper action or prosecution for the enforcement of such penalty, forfeiture, or liability.]

The U.S. Sentencing Commission, on the other hand, is statutorily authorized to decide whether to retroactively apply a reduction within the recommended sentencing range. [28 U.S.C. § 994(u)] Sentencing courts are precluded by statute from applying a guideline amendment retroactively unless the Commission had specifically designated such amendment for retroactive application. [18 U.S.C. § 3582(c)(2)]

There's also some variation within US state laws on this:

States have different approaches to retroactivity. Some have statutes that provide for retroactive amelioration, while others leave it to the discretion of the legislature. California’s Supreme Court has found that legislative intent to make ameliorative changes retroactive can be imputed. [citing: In re Estrada] Illinois, Iowa, Kentucky, New Hampshire, New Jersey, Ohio, Texas, Vermont, Virginia, and West Virginia all have statutes that provide for beneficial changes of law to apply prior to final judgment. Other states that do not have such a provision leave this decision up to the legislatures. In 2006, the Colorado Legislature repealed a 1991 law that created life sentences without parole for juveniles. [H.B. 06‐1315, 65th Gen. Assem., Reg. Sess. (Colo. 2006)] The Legislature, however, explicitly did not make it retroactive for the 48 people in Colorado who are serving sentences of more than 40 years or life in prison without parole for crimes committed during the 15-year window before the change in laws.

But even the California exceptionalism had its limits:

In the absence of a specific legislative mandate, the California Supreme Court has found that legislative intent to apply ameliorative changes retroactively can be implied. “This intent seems obvious, because to hold otherwise would be to conclude that the Legislature was motivated by a desire for vengeance, a conclusion not permitted in view of modern theories of penology . . . When the Legislature amends a statute so as to lessen the punishment it has obviously expressly determined that its former penalty was too severe and that a lighter punishment is proper as punishment for the commission of the prohibited act.”

The Court condemned a purely retributive approach but stopped short of imputing the legislative intent to apply ameliorative changes retroactively to offenders in the post-conviction phase. In People v. Rossi, the California Supreme Court held retroactive amelioration should apply in pending criminal proceedings and prior to final disposition in front of the court with final authority.” In both the California Supreme Court cases, the Court makes clear that a failure to apply such changes retroactively is driven by invalid penological justifications. Yet, the Court states that retroactive application of the beneficial law ends upon final conviction. This would mean that those prisoners who have exhausted their appeals would be left to serve their sentence without benefiting from a statutory penalty reduction unless specifically mandated by the Legislature. Under the Court’s own reasoning, allowing post-conviction offenders to continue serving these sentences would be motivated by vengeance.

Also of interest are international conventions that the US has ratified in this regard, the International Covenant on Civil and Political Rights in particular, although the US has attached a reservation to the relevant article:

The international treaty law governing retroactive amelioration is explicit and clear. The major governing human rights treaties allow an offender to benefit from a change in law that imposes a lighter penalty than the one in existence at the time the offense was committed. Article 15 of the ICCPR, to which the United States is a party, contains a provision that prohibits criminal ex post facto laws noting however, “[i]f, subsequent to the commission of the offence, provision is made by law for the imposition of a lighter penalty, the offender shall benefit thereby.” The United States is one of 167 countries that are party to the ICCPR. The ICCPR sets the main legal framework for the lex mitior principle under which the countries of the world have fashioned their constitutions and penal codes. There has been little dispute from the international community about this section of Article 15. Germany attached a reservation indicating that a lighter penalty would not be provided in certain instances. Nonetheless, Germany enshrines the right to retroactive application of ameliorative law prior to final judgment in its criminal code. Italy and Trinidad and Tobago specified that this right would only apply to cases still in progress or prior to a final judgment. The United States is the only country that has attached a reservation indicating that this section of the Article would not apply under any circumstance. The reservation states “[t]hat because U.S. law generally applies to an offender the penalty in force at the time the offence was committed, the United States does not adhere to the third clause of paragraph 1 of article 15.” However, Article 4(2) of the ICCPR lists this Article as non-derogable, which means that it may not be reduced in any way.

Answer 2

Article 1 Section 9 of the US Constitution forbids ex post facto laws, as do most state Constitutions.

No Bill of attainder or ex post facto Law shall be passed.

Article 1 Section 10 of the US Constitution also forbids States from passing ex post facto laws.

No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.

In a letter to Isaac McPherson, dated August 13, 1813, Thomas Jefferson wrote this about the matter.

The sentiment that ex post facto laws are against natural right is so strong in the United States, that few, if any, of the State constitutions have failed to proscribe them. The federal constitution indeed interdicts them in criminal cases only; but they are equally unjust in civil as in criminal cases, and the omission of a caution which would have been right, does not justify the doing what is wrong. Nor ought it to be presumed that the legislature meant to use a phrase in an unjustifiable sense, if by rules of construction it can be ever strained to what is just.

In Federalist No. 44, James Madison wrote a defense for the outlawing of bills of attainder and ex post facto laws.

Bills of attainder, ex-post-facto laws, and laws impairing the obligation of contracts, are contrary to the first principles of the social compact, and to every principle of sound legislation. The two former are expressly prohibited by the declarations prefixed to some of the State constitutions, and all of them are prohibited by the spirit and scope of these fundamental charters. Our own experience has taught us, nevertheless, that additional fences against these dangers ought not to be omitted. Very properly, therefore, have the convention added this constitutional bulwark in favor of personal security and private rights; and I am much deceived if they have not, in so doing, as faithfully consulted the genuine sentiments as the undoubted interests of their constituents. The sober people of America are weary of the fluctuating policy which has directed the public councils. They have seen with regret and indignation that sudden changes and legislative interferences, in cases affecting personal rights, become jobs in the hands of enterprising and influential speculators, and snares to the more-industrious and less-informed part of the community. They have seen, too, that one legislative interference is but the first link of a long chain of repetitions, every subsequent interference being naturally produced by the effects of the preceding. They very rightly infer, therefore, that some thorough reform is wanting, which will banish speculations on public measures, inspire a general prudence and industry, and give a regular course to the business of society.

. . .

The establishment of the writ of habeas corpus, the prohibition of ex-post-facto laws, and of TITLES OF NOBILITY, TO WHICH WE HAVE NO CORRESPONDING PROVISION IN OUR CONSTITUTION, are perhaps greater securities to liberty and republicanism than any it contains. The creation of crimes after the commission of the fact, or, in other words, the subjecting of men to punishment for things which, when they were done, were breaches of no law, and the practice of arbitrary imprisonments, have been, in all ages, the favorite and most formidable instruments of tyranny. The observations of the judicious Blackstone, in reference to the latter, are well worthy of recital: "To bereave a man of life, Usays he,e or by violence to confiscate his estate, without accusation or trial, would be so gross and notorious an act of despotism, as must at once convey the alarm of tyranny throughout the whole nation; but confinement of the person, by secretly hurrying him to jail, where his sufferings are unknown or forgotten, is a less public, a less striking, and therefore A MORE DANGEROUS ENGINE of arbitrary government." And as a remedy for this fatal evil he is everywhere peculiarly emphatical in his encomiums on the habeas-corpus act, which in one place he calls "the BULWARK of the British Constitution."

Basically, Madison justifies the proscription of ex-post-facto laws as necessary for upholding the social contract between the government and its people and staving off tyranny. Ultimately, much of the US Constitution was written with the prevention of tyranny in mind, and ex post facto laws were considered an "instrument of tyranny."

In regards to lex mitior, an early Supreme Court case seems to uphold it. Calder v. Bull, 3 U.S. (3 Dall.) 386 (1798) found that the ex post facto clause in the Constitution applies to criminal law with at least one of these four effects.

1. Every law that makes an action done before the passing of the law, and which was innocent when done, criminal; and punishes such action.
2. Every law that aggravates a crime, makes it greater than it was, when committed.
3. Every law that changes the punishment, and inflicts a greater punishment, than the law annexed to the crime, when committed.
4. Every law that alters the legal rules of evidence, and receives less, or different, testimony, than the law required at the time of the commission of the offence, in order to convict the offender.

So, under US case law, although the Constitution prohibits ex post facto laws specifically, the courts have held to the standard of a less strict form of lex mitior in general. However, this is only in regards to whether or not a law can legislate against prior behavior. It has no standing in regards to whether or not a change of law should necessarily be applied to past behavior.

For instance, in New Mexico, the death penalty was abolished in 2009, but people who committed capital offenses prior to this change in law could still be subject to the death penalty. This is because the law was specifically not retroactive. If the Constitution mandated lex mitior, I believe that the abolition of the death penalty would necessarily be required to apply retroactively whereas under the US interpretation of the prohibition of ex post facto laws, it would merely be legal for New Mexico to have crafted the legislation so that it applied retroactively.

So, ultimately, the prohibition of such laws is grounded in our Constitution. The Courts have interpreted this prohibition under a lex-mitior-lite interpretation wherein it is merely legal for legislation to make punishments for prior actions less severe rather than necessary to interpret legislation in that manner. As for what this means in the current context and how it has developed into the present, Fizz's answer is sufficient. I merely wanted to point out that the reason that ex post facto laws are prohibited in American jurisdictions is because our founding document, the US Constitution, particularly demands it, and that this provision was justified by those who wrote the Constitution.