Several factors. 1. If you go to prison, innocent or otherwise, you're not getting hired afterward. This leads to desperation, desperation leads to... 2. In p

Several factors. 1. If you go to prison, innocent or otherwise, you're not getting hired afterward. This leads to desperation, desperation leads to... 2. In prison, you become socially reliant on criminals to simply get by. This results in cultural transference. Someone getting in trouble for shoplifting learns about the joys of [insert more lucrative criminal activity here]. Combine this with point 1 and you get repeat offenders who become 'hardened' criminals. Basically, they get better at crime, because crime involves skills just like everything else in life. 3. Poor prison conditions result in psychological trauma. This, in turn, leads to anti-social behavior. In short, we pay a lot of money to create better criminals who are highly motivated to commit more crimes.