Im not sure, but i think its because if you tell a troll that he was baned its very likely that he would make another account and continue with his previous act

Im not sure, but i think its because if you tell a troll that he was baned its very likely that he would make another account and continue with his previous actions, but if he doesn't know, he'll simply think he is being ignored. Its literally preventing trolls from being fed. Im not sure if HN does that.