The rule about not being allowed to call a troll a troll

[JWeinCom]

To play devil's advocate... what good does calling a troll a troll do? 

If you call someone a troll do they say "Oh damn they got me I'll shut up now"? More likely than not, they'll just troll harder and further derails the conversation.

It doesn't really protect trolls, because being the center of attention is what they want, even if it's for trolling. I imagine the theory is to dissuade people from feeding the trolls, and make them more likely to report than to try and engage.