One could make the case that they are in fact volunteering for their servitude.
There are strict criteria, which you have to go out of your way to meet, that determine whether you're eligible to get in to a prison. It's easy to make the argument that everybody there is there by choice.
Even when guilty, there are many punishments which are unconscionable and amoral to impose for the crimes. For example, stoning a woman to death because she talked to a man who is not a relative. Another is third strike life in prison for smoking pot.
There are a lot of people that are railroaded into prison for simply being in the wrong place, looking like the wrong guy, or simply having a bad attitude. In some states it's extremely arbitrary, where they will brand you an accomplice simply because you picked up a hitch-hiker who happened to have drugs on them.
Just for a single counterexample: anyone who has ever been falsely accused of rape. You can literally spend your entire life sitting at home watching TV, and end up in a prison.
There are strict criteria, which you have to go out of your way to meet, that determine whether you're eligible to get in to a prison. It's easy to make the argument that everybody there is there by choice.