The US prison system is messed up, but mostly with regard to nonviolent/drug offenders. I am very libertarian, but I don't think this is a valid criticism in this case. Robbing a bank is not "making a mistake once". Buying coke or shoplifting is making a mistake. Bank robbery is a premeditated, violent, serious crime, and society is right to punish it severely. If you're committing bank robbery, you're likely also committing lots of other crimes: assault, threats, criminal confinement, fleeing police, etc.