The shameful aspect is due to the wide differences in how states choose to apply this rule and how many people are barred from voting even though they served their time, sometimes for a single event or minor offenses, and have not been convicted of later crimes. I'll give a few examples. You don't need to point out again that states are allowed to do this, or challenge my opinion that I think it needs to be changed. You have already done that.
Except for Wyoming, all states with over 5% voter disenfranchisement are in the south, and all disproportionately use these laws to target blacks. All states with high percentages have conservative and Republican legislatures and Governors. About 2.5% of all potential voters in the country can't vote, but that includes 7.6% of blacks. It's obvious that even if blacks disproportionately are convicted of crimes (another racist issue), the laws allow states to suppress black voting.
Florida's rules have flip-flopped from one extreme to another over the past 8 years. The current policy was instituted with no clear reason, other than to be punitive to ex-cons who would most likely vote for Democrats more often than for Republicans. That covers over 1.5 million people in Florida, or about 10% of the population, and a mind-boggling 23% of the black population. Really? It's clear that if the state ever swings Democratic (which is harder to do with so many potential Democratic voters barred from the polls), the law will be reversed again. If that happens, it's likely the state will never elect a Republican Governor or Senators again, which is why todays Republican government of the state has such a strict law. This is the nastiest kind of Jim Crow in the country, and as you point out, it's allowed by the Constitution.
Florida - On Mar. 9, 2011 the Florida rules of Executive Clemency were toughened. Automatic restoration of civil rights and the ability to vote will no longer be granted for any offenses. All individuals convicted of any felony will now have to apply for executive clemency after a five year waiting period. Individuals who are convicted, or who have previously been convicted, of certain felonies such as murder, assault, child abuse, drug trafficking, arson, etc. are subject to a seven year waiting period and a clemency board hearing to determine whether or not the ability to vote will be restored.
Prior to the Mar. 9, 2011 rule change some individuals convicted of nonviolent felonies were re-enfranchised automatically by the Clemency Board upon completion of their full sentence, including payment of fines and fees.
Kentucky similarly recently reversed the policy for no apparent reason. That state also can withhold voting privileges of people who have been convicted of "high misdemeanors", but as best I can determine that term means that the person was convicted of a crime that made them "infamous".:
Kentucky - On Nov. 24, 2015, Kentucky Gov. Steven L. Beshear issued executive order 2015-871 to automatically restore the right to vote to nonviolent felons who have completed probation, parole, and who have no outstanding court-ordered restitution payments. On Dec. 22, 2015, newly elected Gov. Matthew G. Bevin issued executive order 2015-052, rescinding the previous Governor’s executive order.
As a result, people convicted of any felony in Kentucky must individually apply with the Governor to have their voting rights restored.
In Hunter v. Underwood (1985), the SC ruled in favor of two men who were convicted in Alabama of "moral turpitude" for trying to cash a bad check. Amazingly, when they first sued in Federal Court, they lost with the decision noting:
Edwards and Underwood sued the Board of Registrars at a Federal District Court, which found that indeed the outspoken purpose of the constitutional change was "the disenfranchisement of blacks", but could not find it proven that this was based on racism, and decided against the plaintiffs.
The south is so twisted when it comes to matters of black repression that it's possible to believe that systematic attempts to deny blacks the right to vote is somehow not due to racism.