The problem here is that when you let someone punish you, you aren't really being punished. Coercion is a key aspect of punishment. Without it, "punishment" is rarely corrective. Sure, it may have made Zack feel better, but it didn't really do much for his behavior in itself. That means the beating had no value for Kate and the others. Hell, Zack even graciously told his captors that he'd stay imprisoned until Kate decided he should come out. As punishment, it was a joke. That doesn't mean it didn't have value for anyone, but it wasn't what she was really after either: evidence that she can correct his behavior through violence. What corrected his behavior was her vow to hate him if he abused his victims again (emotional coercion). I get that for some people the beating would be required for that message to get across that the person was serious, but Zack demonstrably isn't one of those people -- since, you know, reads Shadows.
"If trees could scream, would we be so cavalier about cutting them down? We might, if they screamed all the time, for no good reason." --Jack Handey