If someone throws a nickle at me I am not justified in a court of law if I attack them. It is called proportionate response.
Now I am also looking at what he did first and I am coming to the conclusion that this guy liked humiliating little girls or at least this girl.
"Can't you count?"?
Would you ever say that to a customer? Maybe if you were a smart mouthed teen on your first job. But not when you are a 44 year old business owner.
And you are correct, there is a double standard. He was expected to behave like a grownup. He didn't.
Bad customer service is not an excuse for assault, if she cannot deal with mean people she should not be in a store by herself. Once she threw the money and spat on him she crossed the line of proportionate response for bad service.
Again equal justice for all. Both have done wrong and both should be punished by the law, anyone calling for less is wrong.