I thought voting was a right, enshrined in the US constitution? And yet you take away that right when they're locked up? Seems kinda ass-backwards to me, but okay.
But seriously, they absolutely should be allowed to vote. Even while in prison. The only reason you have to remove someone's right to vote is if they're incapable of making a rational decision due to not being able to properly process the information, like if they're children or mentally handicapped.