It's generally so that the person in question knows they've been hellbanned, as it can be difficult to tell if you have been without someone mentioning it.
The purpose of hellbanning is precisely so the person in question doesn't know.
The purpose of hellbanning is to prevent people from ruining the site. How exactly josephcooney was ruining HN by posting "The question didn't seem rude to me at all, either. Why is plain communication so often taken as cynicism/sarcasm?" is a mystery to me.
I don't know why he was hellbanned; I was just explaining the purpose of hellbanning. FWIW after skimming through josephcooney's history, I am also mystified as to why it happened.
The first thing that came my mind is a mod or bot might have seen his handle and considered it a troll account b/c of it's similarity to Joseph Kony
emotional responses by weird aspies is not particularly surprising to me