From the newsgroup: non-cursed dunce cap or helm of opposite alignment
which would glow black and become cursed if worn by the hero wasn't doing
the same when worn by a monster. Fixing this gives players a new way to
recognize bad hats--drop on altar, then drop in front of approaching goblin
or other wimpy monster capable of wearing armor--but I think that's fair.
Since helms of opposite alignment are usually already cursed it won't make
much difference for them; most players seem to avoid all conical hats so it
won't make much difference for dunce caps either.