In some states in the US convicted felons cannot vote, hold public office or serve on a jury, even though they completed their sentences many years ago and have not reoffended since. This seems disproportionate; surely the purpose of the justice system is to straighten the offender out and then reintegrate them with society, not create a permanent barrier.
What are the arguments in favour of such laws?
The main reason is a corrupt one: to increase and maintain the political power of a (tyrannical) ruling party. It can outlaw customs and traits specific to opposing parties, and jail their voters. Those opposition party citizens can never again vote against the ruling party.
The customs and traits might be, (and have been), anything at all. Religions, economic views, hobbies, fashions, musical or literary tastes, recreations, celebratory customs, ancestry, income, assets, etc.
Evidence: Corrupt laws are invariably enacted with legal fig-leaves and disguises with which to assure the public that the bad laws in question are merely to prevent some remotely genuine (but vastly overestimated) evil. There may be smoking guns left around, but the most usual evidence is historically informed comparison and inference, along with a working knowledge of why a corrupt law is unnecessary for its purported aim, and why it was sponsored, and who sponsored it, and who wants to keep it going.
An underlying assumption of such laws is that a citizen's vote is a kind of "privilege", granted as a reward for compliance and conformity by a jealous and watchful sovereign state, and that this state therefore would remain healthy even if not one citizen deserved or was granted the reward of voting. Voting in this light is something any citizen does mainly for their own advantage.
Disenfranchising laws never regard general voting as being necessary for the health of the democratic state, a function more important than any motivational perk for the citizen. Or that citizens votes are to the body politic of a democracy what the somatosensory system is to a healthy human body. Just as a person with injured or impaired somatosensory cells is less able to avoid bodily injury, so too a democracy deprived of some subset of its voters is less able to avoid national injury.
A implies B for B implies A.
Strictly speaking, it would generally not be laws (statutes), but a state constitution which governs voting qualifications, but statutes may clarify the implementation of the constitution. For example, article II of the Iowa constitution deals with "suffrage", saying (among other things):
Disqualified persons. Section 5. No idiot, or insane person, or person convicted of any infamous crime, shall be entitled to the privilege of an elector.
Thus by that wording, voting is considered a privilege, not a right. Note that constitution does not define any of the terms "idiot", "insane person" or "infamous crime". In cases like these, courts get called on to interpret the constitution, and in 2016 the Iowa Supreme Court did just that in Griffin v. Pate, saying that:
This appeal requires us to decide if the crime of delivery of a controlled substance is an “infamous crime” under the voter disqualification provision of the Iowa Constitution. The district court held the crime is an infamous crime, and a conviction thereof disqualifies persons from voting in Iowa. Following the analysis we have used in the past to interpret provisions of our constitution, we agree and affirm the judgment of the district court.
The term “infamous crime” was generally recognized to include felony crimes at the time our constitution was adopted. This meaning has not sufficiently changed or evolved to give rise to a different meaning today. In addition, unlike some past cases when we have interpreted provisions of our constitution, the facts and evidence of this case are insufficient to justify judicial recognition of a different meaning. Constrained, as we must be, by our role in government, we conclude our constitution permits persons convicted of a felony to be disqualified from voting in Iowa until pardoned or otherwise restored to the rights of citizenship. This conclusion is not to say the infamous-crime provision of our constitution would not accommodate a different meaning in the future. A different meaning, however, is not for us to determine in this case. A new definition will be up to the future evolution of our understanding of voter disqualification as a society, revealed through the voices of our democracy.
Nothing in this interpretation prevents the Iowa legislature from more precisely defining which crimes are "infamous", nor does the that constitution say that the disqualification is permanent, only indefinite; in Iowa, a convicted felon can seek restoration of his voting privileges by applying to the governor.
An official statement after the verdict said:
"When I took the Oath of Office in January, I swore to support the Constitution and the integrity of Iowa's elections. That includes efforts to insure elections are not diluted by ineligible voters." - Paul D. Pate, Iowa Secretary of State
Please note I'm not supporting or opposing secretary of state's position on this, just attempting to addressing the question of why: because the drafter's of that's state's constitution chose to consider some people disqualified to vote. If you want more detail on the arguments, there is a video of oral arguments available.
