If you're in jail, you lose privileges. One of those should be voting privileges.
There is a reason for this. With thousands of people of voting age in jails, those numbers could swing close elections, and having politicians pander to prisoner special interests for their votes threatens the integrity of the public offices.
It prevents the formation of a prisoner voting bloc, and you know that would happen.
And the U.S. would be turned into one big state of New Jersey!