Perhaps the dude with the blown-apart bum didn't know a horse often has a rather sizable dink? Maybe his human buddy told him he was hung like a horse and just assumed horses were that size. He had his back turned and probably never saw it coming.
I suppose it isn't illegal but that's a moot point now. The immoral and stupid act was punished enough and should act as a deterrent for all but the biggest assholes.