According to this article, Canada would be breaking at least three UN treaties once it legalized marijuana in October. But why didn't the Canadian government just leave the treaties beforehand? It seems like a painless process with no repercussions, and they could join back any time they wanted if necessary.
Probably because Uruguay didn't do it either and apparently without consequences:
While Canada appears set to violate these international agreements, the [International Narcotics Control] board has limited powers when it comes to forcing signatories to comply.
Gélinas-Faucher said that the agency can ask countries which signed these agreements to discontinue their drug trade with Canada, since the agreements govern trade in legal drugs as well. And another country could sue Canada before the International Court of Justice.
Those scenarios, however, are considered unlikely. Uruguay, which also legalized cannabis, has not been subjected to similar reprisals.
And here's what happened in that case, basically Uruguay argued that human rights trump drug enforcement:
Uruguay has forged ahead with cannabis regulation despite repeated criticisms from the International Narcotics Control Board (INCB), as in the Board’s report for 2016, which states:
The Board notes the continued implementation by the Government of Uruguay of measures aimed at creating a regulated market for the non-medical use of cannabis. While this policy has not yet been fully implemented, the Board wishes to reiterate its position that such legislation is contrary to the provisions of the international drug control conventions, particularly to the measures set out in article 4, paragraph (c), of the 1961 Convention as amended, according to which States parties are obliged to ‘limit exclusively to medical and scientific purposes the production, manufacture, export, import, distribution of, trade in, use and possession of drugs.’
In opting for regulation notwithstanding such criticisms, Uruguay has argued that its policy is fully in line with the original objectives that the UN drug control treaties emphasized but have subsequently failed to achieve: namely, the protection of the health and welfare of humankind. While there can be little doubt that, as the INCB has pointed out, Uruguay is contravening its obligations under the 1961 Single Convention to limit cannabis exclusively to medical and scientific purposes, Uruguay has sought to side step the question of drug treaty non-compliance by placing its new law in the context of the country’s adherence to its more foundational obligations under international law.
Uruguayan authorities have specifically argued that the creation of a regulated market for adult use of cannabis is driven by health and security imperatives and is therefore an issue of human rights. As such, officials point to wider UN human rights obligations that need to be respected, specifically appealing to the precedence of human rights principles over drug control obligations within the UN system as a whole. In the event of a conflict between human rights obligations and drug control requirements, they argue, Uruguay is bound to give priority to its human rights obligations.
The argument for the priority of human rights obligations in matters of drug control is not a new one for Uruguay. In 2008, Uruguay sponsored a resolution at the Commission on Narcotic Drugs (the UN’s central policy making body on the issue) to ensure the promotion of human rights in the implementation of the international drug control treaties. Uruguay’s argument that human rights protections take precedence over drug control requirements finds support in the 2010 report to the UN General Assembly by the UN Special Rapporteur on the Right to Health, which signaled: “When the goals and approaches of the international drug control regime and international human rights regime conflict, it is clear that human rights obligations should prevail.”
In 2015, Uruguay co-sponsored a resolution calling upon the UN High Commissioner for Human Rights (OHCHR) to prepare a report ‘on the impact of the world drug problem on the enjoyment of human rights.’ In its contribution to OHCHR preparations, Uruguay laid out its stance regarding the primacy of human rights:
We reaffirm the importance of ensuring the human rights system, underscoring that human rights are universal, intrinsic, interdependent and inalienable, and that is the obligation of States to guarantee their priority over other international agreements, emphasizing the international drug control conventions.
Uruguay’s ability to move forward with a policy clearly beyond the bounds of the UN drug treaties owes to a combination of factors. First, Uruguayan authorities foresaw the international criticism their move would likely trigger, and fashioned an argument based on human rights obligations that was consistent with the country’s international reputation, and that was coherent with the country’s rationale for revising its cannabis law in the first place. Second, as a practical matter, the UN drug control treaty bodies, including the INCB, do not have the kind of enforcement authority or practical political power that would be necessary to prevent Uruguay from moving ahead with implementation of its new law. Countries such as the United States have historically wielded their political influence and power to encourage full implementation of the drug treaties. However, with Uruguay’s law entering its fifth year since passage, there has not been a concerted U.S. government effort to punish Uruguay bilaterally or in an international arena, suggesting that Uruguay’s reforms will not be stymied because of international pressures. In this regard, Uruguay has taken advantage of felicitous timing, with its law’s passage having come in the midst of a major shift toward cannabis regulation within the United States. After the November 2012 ballot initiatives to legalize cannabis in the states of Colorado and Washington, U.S. President Barack Obama’s administration adopted a policy of conditional accommodation of state-level cannabis legalization, contained in Justice Department enforcement guidance known as the “Cole Memo.” This accommodation provided Uruguay a political cushion internationally, just as the Uruguayan parliament was preparing to approve the country’s cannabis reform.
In the wake of the Colorado and Washington ballot initiatives, the U.S. federal government was suddenly in an awkward spot. The United States was the key architect and for decades the chief enforcer of the UN drug treaties, including vigorous enforcement of the global prohibition on non-medical uses of cannabis. To oppose Uruguay’s new law or even pressure Uruguay to revise or annul it—as it is easy to imagine previous administrations attempting to do so—would open the United States to charges of hypocrisy.
Indeed, regarding non-medical cannabis, the INCB has also repeatedly noted that the United States is “not in conformity” with the drug treaties, and has underscored that the “strict prohibition of non-medical use set out in the 1961 Convention” applies fully to countries with federal structures of government. In other words, if “sub-national Governments have taken measures towards legalizing and regulating the non-medical use of cannabis, despite federal law to the contrary”—as is quite evidently the situation today in the United States—then such developments are “in violation of the international drug control legal framework.” In this new context, the United States has kept its criticisms of Uruguay’s cannabis law soft and perfunctory.
Under President Donald Trump, U.S. Attorney General Jeff Sessions has made clear his animus toward legal cannabis. In January 2018, Sessions rescinded the Cole Memo, heightening concerns over how federal enforcement powers will be wielded. But Sessions’ bid to turn back the clock on cannabis legalization is considered unlikely to succeed; cannabis reforming states are not expected to be reverse course, even if the Trump administration expends significant political capital in an effort to compel them. This leaves the U.S. federal government in the same awkward situation that began in November 2012 when the voters of Colorado and Washington State approved their ballot initiatives: unable to undo the states’ reforms, and therefore out of compliance with the drug treaties it has long championed. For the foreseeable future, the United States is unlikely to be in a position to oppose efforts to legalize and regulate cannabis, such as that now underway in Uruguay.
It looks like Canada is taking a somewhat similar line (to Uruguay's)...
Witnesses appearing before your Committee all agreed that if Bill C-45 is passed, Canada would be in violation of the 1961 Single Convention, the 1971 Convention on Psychotropic Substances, and the 1988 Anti-Trafficking Convention.
Your Committee heard from the Minister of Foreign Affairs at the end of its hearings. In her testimony, the Minister “recognize[d] that [the] proposed approach of legalizing, [regulating], and strictly restricting cannabis will result in Canada contravening certain obligations […] under the three UN drug conventions: the Single Convention on Narcotics Drugs from 1961, the 1971 Convention on Psychotropic Substances and the 1988 United Nation Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances.” Even as Bill C-45 will violate certain international obligations related to cannabis, the Minister of Foreign Affairs noted that Canada’s policy regarding international drug control remains consistent with the overarching goal of the three conventions, namely to protect the health and welfare of society. Moreover, while GAC officials in earlier testimony characterized the contravention of the international drug conventions as technical, the Minister of Foreign Affairs stated before your Committee that “the issue of the conventions is an important one, and [the government] need[s] to be clear about it.” [...]
She informed your Committee that government officials have been discussing Canada’s intention to legalize recreational cannabis with G7 countries as well as a dozen others. In particular, she noted that Canada was re-elected to the Commission on Narcotic Drugs (CND) in a contested election in March 2017, after having announced its intention to legalize recreational cannabis.
In answering questions put to her about how Canada intends to reconcile its views on the importance of a rules-based international order with Bill C-45’s violation of international treaties, the Minister pointed out the need for Canada to be open about being in contravention, to preserve the health and safety of Canadians, as well as to work with international partners.
And yes, INCB also protested against Canada in similar terms as against Uruguay; I'll spare you the quote. Their position will probably have no effect.
The Canadian discussions also covered various solutions.
One was to re-accede with a reservation like Bolivia:
Your Committee heard about the experience of Bolivia. According to Mr. Jelsma, in January 2012, Bolivia withdrew from the 1961 Single Convention after failing to obtain a reservation from the criminalization provisions of the treaty for the traditional use of the coca leaf, which is protected in the Bolivian constitution. A year later, the country was able to obtain a reservation and re-acceded to the treaty.
Another suggestion was a convention between the cannabis-legalizating countries:
Some witnesses discussed the option of inter se modification and the role Canada could play in its pursuit. Your Committee was informed that the inter se option would entail the negotiation of a side agreement on cannabis among like-minded countries that are parties to the three drug control conventions. It would also maintain “a clear commitment to the original treaty aim to promote the health and welfare of human kind, and to the original treaty obligations vis-à-vis countries that are not party to the inter se agreement.” This option is provided for in Article 41 of the Vienna Convention on the Law of Treaties.
And there's some Canadian precedent for going that route:
Professor de Mestral pointed to Canada’s experience with the 1970 Arctic Waters Pollution Prevention Act, which asserts the Government of Canada’s jurisdiction over the navigation of international shipping through Arctic waters. He noted that the Act generated an “extremely negative” reaction on the part of the international community, including several protests from allies, as it was in apparent contravention of international law governing freedom of the seas. In drawing conclusions about this experience, Professor de Mestral noted:
We built a consensus, the third Convention on the Law of the Sea gave us a platform where we were able to make our arguments successfully that change was needed. But clearly in the face of protests, Canada simply said, “We will not go to the international court on this issue. We believe we are right. We believe that change has to be made to international law, and we’re prepared to do it with like-minded states. I guess that may be what Canada will have to do.”
However, with regard to cannabis, the Canadian government [for now] seems undecided what [else] to do, beyond openly admitting its [technical] non-compliance.
Because such treaties aren't quite as binding as they may seem at first glance, and helpfully include severability clauses intended to allow a little wiggle room.
First an analogy:
Civil law contracts and agreements drawn up unilaterally often throw in a pile of "gimme" clauses that seem to require, demand, or imply that many more things be given than the signers actually legally must provide. A slight exaggeration:
Rumpelstilzchen Inc. also requires the firstborn child of signer within 10 days of birth via FedEx courier for processing to be sold as unguents... this is a binding contract ... bla bla bla ... These Terms are void where prohibited by law
Of course the firstborn child requirement is prohibited by law and therefore not legally binding, and therefore is void, but every now and then some signer who didn't read far enough innocently provides Rumpelstilzchen Inc. with free feedstock anyway, so it's sometimes profitable for the drafters of contracts to leave all sorts of junk clauses in, followed by an overarching void clause.
End of analogy.
For treaties, there are similar catch-all clauses roughly equivalent to "void where prohibited by law". For example, the Single Convention on Narcotic Drugs, has the catch-all clause "Subject to its constitutional limitations":
Article 36: PENAL PROVISIONS
1. (a) Subject to its constitutional limitations, each Party shall adopt such measures as will ensure that ... bla bla bla ...
...and anything in that list of provisions which conflicts with Canada's Constitution is not binding, or at least would be subordinate to its Constitution.
And so on for all treaties with similar void clauses...
Also some nations think it better to follow the general spirit of a treaty, rather than the letter of it should the letter tend to weaken the treaty's purpose. In 2018 still classifying cannabis as a narcotic seems like dubious and obsolete pharmacology. It's arguable that it's better to only just ignore any items erroneously still included, in lieu of ignoring or throwing out an entire treaty just because it contains any single mistake.