Abusing prisoners falls under the category of unfairly damaging or killing the prisoners when they have no control over the game or the outcome. As Medviah stated for duck hunt, CTs shooting prisoners as they're jumping off a platform and their only saving grace is luck and surviving the landing if they weren't killed in mid air, is not fair. That is completely out of every prisoner's control and the CTs are basically target practicing on everyone.
Abusing prisoners also includes, but is not limited to: Baiting prisoners, getting knifed, killing said prisoner, free damaging or killing prisoners before they get to a certain location before the stated time by Warden, telling a prisoner to do a certain command and killing him anyways, etc, etc, etc. It's pretty self explanatory. Use common sense when in Jail and being a guard.