Right, but you either need to pay someone to do the torturing, or you have to do it to them yourself. If the former, how do you force that person to go along with it, instead of just rising up against you and taking your place? It only takes one guard to realise that with a bit of help from the invincible prisoners, they can successfully mutiny.

Invincibility (which is what I am asserting “no death” effectively is (open to interpretation)) effectively nullifies the threat of violence, which, when all else fails, is the only thing keeping things ordered.