They associate undesirable behavior with pain. Most kids try to avoid pain, hence they'll learn to avoid whatever it is that lead you to discipline them.
They associate pain with their parents. This is bad because ones' relationship with their parents tends to be a template for their relationships with other people of the respective genders.
And sometimes he is. Just because she's an adult doesn't mean she's incapable of making mistakes.
Does that mean that since I percieve you to be making a mistake in the way you see education, it's OK for me to beat you? Adults make mistakes, after all.