In prisons, those things withheld from and denied to the prisoner become precisely what he wants most of all.
History is full of people who went to prison or were burned at the stake for proclaiming their ideas. Society has always defended itself.
Whatever you think of de Sade, he was a complex figure and we should not look for easy answers with him. He was, strangely perhaps, against the death penalty, and he was never put in prison for murders or anything like that.
When I was in prison, I was wrapped up in all those deep books. That Tolstoy crap - people shouldn't read that stuff.