These mobs appear when somebody is suspected of using a third-party program or bot, or killing too fast that might mean cheating.
If it was a bot they would hit it and get disconnected. If it's a person and sees Do Not Attack, they know what to expect, they shouldn't attack.
P.S. I -did- attack when I saw it the first time, ofc I did. Just couldn't resist, I hate it when somebody tells you "do not do that". :P