How would you react if you saw a mom punch her baby?

Apparently there's this prank going around where you pretend to abuse a baby. I really don't understand this new age if pranks. First of all if I saw a woman punch her baby, I would beat her face in. Second how are child abuse pranks funny? There are actually parents out there who abuse their kids. There are actually babies beiny abused and neglected by their parents. Someone please explain how pretending to abuse a baby in public ia funny?