It's too bad that a little girl felt shamed or embarrassed because she unwittingly made an inappropriate gesture -- or something that looked like it. (Done one way, the gesture is sexual. The other way, it's vaguely Satanic.) However, the gesture in question is inappropriate, regardless of whether its origins are widely known, and I don't think the CM did anything wrong by pointing that out. (I'm guilty of this kind of "innocence," too -- as a student teacher in college, I once permitted some middle schoolers to recite the word "" in a poem they were reading aloud, because I thought it was simply a nonsense word. Of course, as I was later informed BY AN ELEVEN-YEAR-OLD, it's not -- it's a contracted form of an expletive. My ignorance was no excuse, as I should have investigated the unfamiliar word ahead of time.)
Bottom line: if someone says that a gesture your child is making is inappropriate and you don't know what it means, then it's time to do some research so you can instruct your child accordingly. If a poster spelling out what the gesture meant is offensive to us, then imagine how offensive it must be for CMs to see children doing it, and being expected to sell the photographic evidence.