Prison being shitty covers retribution and deterrence. Prevention is provided merely by having strong walls and razor wire.
If somebody is locked in a cell, then they will be physically unable to rob the local liquor store. How well or poorly they are treated is irrelevant in that regard.
Why not let the cops simple shoot the man once convicted ? Isn't that retribution, deterrence and prevention all in one ?
People should know a-priori the consequences of their actions. Otherwise it is very likely that there is will be manny shitty prisons and some VIP cool prisons.
If somebody is locked in a cell, then they will be physically unable to rob the local liquor store. How well or poorly they are treated is irrelevant in that regard.