The same idiotic logic can be used to justify a young athlete beating the shit out of an elderly man.
Also, the video clearly shows Johnson grabbing the women prior to her throwing that weak punch. And if you think that this was justification for him punching her in the face, I'm pretty certain that you'd think differently if this happened to your mother, sister, girlfriend, wife or daughter.
Quite frankly, it's less about gender and more about physical mismatch. It's not about what you can get away with per the law, it's about having some god damned sense about how much damage you can do to someone when you are that much larger than them.