Hitting/spanking animals

Okay, so I've seen posts about this topic and the majority of people agree that its NOT okay to hit or spank your animals, and that there are other forms of discipline & positive reinforcements used to teach animals. But, I'm wondering, if that is true, why do animals hit/spank/swat their young to discipline them? Cats and dogs will pop their youngs nose, Mama bears will swat at and dangle their babies even if they're just annoying them, birds scowl at their brood. So why isn't it okay for the owner to hit/spank their animals when trying to dicipline them when it's a common and natural occurrence in wildlife ?