They aren't thinking past their anger. Their intent is to hit the child. The child's instinct is to avoid the blow. So why do the parents tell the child not to put their hands up? I have no idea.
There are other ways to discipline the child other than hitting, however, some folks were raised where the discipline was hitting and unless you're shown another way to discipline or you've actually decided to do something different, they will go the way they were raised.