I like your probation idea, but probation is good for people who are not a high risk to society. I think people we are considering here are be considered for the death penalty which takes them out of that category. Perhaps after some time behind the wall and rehab, probation would be rational step to take.
Now you're coming around. The next fact you need to realize is that there are something like 2 million prison beds in the US. They must be kept full because of the same economic reasons you mentioned before. They need the free labor to keep the prisons running at minimum expense on things like food and medical care. That leaves salaries. You think they want to lay their people off? Hell, no. It's just like any corporation. It takes on a life of its own, self-perpetuates, expands, with budgets, 5-year plans and 10-year plans. You notice how justice has nothing to do with what I'm saying?
Many people want punishment, when separation is all we need between the dangerous types. Why else should it matter what happen on the other side of the wall if we are safe from them? That's the purpose of such a wall, to keep us safe. Right now we just don't think in a way that make it self-sustaining, which is completely practical, would give their lives more meaning, give the best opportunity at compensating society, and give them the best environment for preparation for release.
When is a person considered dangerous? Because he carried a joint into a child safety zone? Because he punched a hot headed cop? Or maybe he forced his girlfriend into the car and took her home to talk to her. All of these are aggravating circumstances that are used to designate these people as dangerous threats to society.
Another issue is this. Today someone is behind a wall because he is labeled dangerous. But tomorrow is sentence is up, and he's coming out. Do you really believe he changed from dangerous to safe in one stride over that threshold? It's ludicrous. If he's still dangerous he should never get out.
What I'm tying to get you to see is that this is all a matter of perception. The only people we can say are actually dangerous are certain classes of psychotic and psychopathic cases, but these need to be diagnosed.
Suppose you have this warm cuddly pet that your kids are crazy about and they pamper it like a baby. But it goes out a kills another animal in a fit of animal blood lust and doesn't even eat it because it's not even hungry. The pet is not considered dangerous. It may gross the kids out but they'll be back to pampering it in no time as if nothing ever happened.
Now you (or any one else here) tell me what's different about the way we think of human dangerousness. It's a perception, one arising out of primal fear and the world of suspicion that grabs the mind and robs it of logic and temperance.
My point is this: before any convict ever goes behind any wall there should be a diagnosis from a competent authority in psychiatry that the convict is dangerous. Trials should be changed, so that once the defendant is found guilty of a crime with a prison sentence, there should be a second trial in which the defendant may prove he is not dangerous, and, if he wins that trial, he gets probation instead of prison.
Like I said before, a small city of a 1000 people in American can be self-supporting and so can (and easier) 1000 people behind a wall. We have to think outside this stupid box we are in to do it though.
I don't know where you get this idea that prisons are not self-supporting. You never heard of prison farms, chain gangs? What do you think they are doing with 2 million slaves to work for them?
Again I say: no one should be put behind any wall without a trial that establishes actual dangerousness, not the mere perception of dangerousness.