I wonder how old most of them are? Kids dressing up to scare other kids, I can understand - nasty, of course, but not that bizarre. If they're adults, though, then they should know better and it serves them right if an irate parent (or granny!) decides to give them a clout.
I don't think my grandchildren in the US are bothered about the phenomenon, and the two little ones in this country are too young to be out by themselves, so it doesn't seem to affect us at the moment...I suspect that any clown who tried to terrorise any of them would seriously regret tangling with their dads.