The truly insane have enough on their plate
A Usenet law that basically says that you shouldn't troll/harrasses/ect people who aren't equipped with the mental tools to deal with it.
Sometimes this is extended to people who are going through a traumatic period, dealing with a terminal illness, death of a loved one or anything else which would remove their better judgement.
Partly this is done because it's realised that such actions are just going to make the victim's life so much worse pushing them closer to total collapse, but mostly it's avoided because it lacks any challenge.
see also: Kicking a cripple.