Straw man.  No one argued that someone can steal a car behind bars.  Are you intentionally being dishonest or did you just miss the point about blatant fearmongering?
But let me address this point.  It seems you want to lock up people merely because of the possibility that they might later commit a crime while not behind bars.  You condemn them for their character rather than any actual criminal action.  By the same logic, why not just kill them since you clearly do not consider any possibility of rehabilitation?  The dead also cannot commit any crime just like a person locked up behind bars.