I think they do that these days because upset anons conflate pedophile and pederast and then apply it to them, and the knee-jerk response is "Nuh uh, just 2D!". In that way pedo became a dirty word. But this rigid connotation didn't start with horny oldfags, as you must be aware.
I wasn't joking when I said our mascot was named PEDObear. That really seems to escape a lot of newfriends when they go on about how people are hiding scurred from the word pedophile. But the new blood insisted on associating the word with highly illegal materials and acts (as opposed to drawing Toph in lingerie) and it stuck! It became the line in the sand, and now, like you, they take girlish glee in seeing people not want to step over it.
Frankly I don't even blame them, or you. Anons are insecure and feel the need to defend themselves all the time, that's part of the problem. You call me a pedo I'll dig up a pedobear image and have fun with you. I wish more people would.
Have some more Peni in your honor.