As near as I can see, the rule came about from the perception that the word was being used as a way to denigrate or insult Americans, in the same fashion as a racial slur. When I read the word being used I never "heard" it that way, myself, but then again, I'm not American. I heard it in the same way that "hoser" or "canuck" is sometimes used to describe us Canadians. In a playfully derogatory way, not in a hurtful way.