Their hope is that maybe they'll be able to change the future for the better by making an example of someone, even if they have to be excessive, cruel, or unjust toward that person. They're justifying the means by the hoped end.
First, the vast majority of states have some type of voting restrictions on felons, with Maine and Vermont being the only exceptions.
Second, some states historically and modernly controlled by the Republican party (e.g. Montana) return the right to vote after release.
Third, among states with harsher restrictions (ending after parole or probation) that are historically OR modernly Republican controlled (ignoring the first point-that many such states are Democrat controlled), the demographics are, in many cases, such that all other races combined could not muster even the perception of a threat to the white, Republican voting bloc.