Wow, the opening post looks so outraged.

Prisonners can vote in France (though many don't bother to). Civil rights are only taken away in case of really serious offenses and state crimes (treason and what not).
Result? Barely 10% of them vote anyway.

Given that the voter turnout is in the US much lower than in France, I expect that barely 5% of your prisonners will vote. Big deal.

And I still don't see any reason why they should be allowed to vote. As I said, some offense should mean you lose your voting rights, but prisonners are still citizens, whether you like it or not.