A conversation with a prison auxiliary helped me understand why. He had transferred prisoners to a private prison. They're reminded that they're people first and prisoners second. Their whole demeanour changes. I had to admit I had never been to a prison like that.
Prison is designed to disempower.
Everyone in jail is vulnerable, to a greater or lesser extent. Prisoners live at the mercy of those in charge, and of each other, and dignity is a scarce commodity. If the regime is accompanied by an attitude that undermines a prisoner's confidence and self-esteem, then all the stated good intentions will be worth nothing. In future, imprisonment will be about warehousing. I witnessed what he meant.