@Rainer -- Re: @AC It's not "why", it's "how"
Have look here at felons and voting. Many, if not most, states do allow felons to vote either after serving their time or after a period time after release.
https://en.wikipedia.org/wiki/Felony_disenfranchisement
https://exoffenders.net/felon-voting-rights/ This site further breaks down the various state rules.