It's the only explanation I could find for why a fist fight between consenting adults is illegal. It encroaches on the government's monopoly on violence.

Also martial arts of all sorts. And hockey. And really any fights in a professional sports capacity gets overlooked. At least a few states have "mutual combat" laws which would theoretically enable you to consentually fight someone as long as it's not also breaking some other law they could hit you with like public disorder.