Broadly speaking, US law is written to pin a battery charge on whoever injures someone, and pins a high burden of proof on someone trying to claim self defense or provocation as a legal defense against that charge. Basically if it can be shown that you hit someone, you're probably guilty of battery unless you have unusually good evidence or an unusually expensive lawyer.

The law was written this way because it was borderline impossible to prove who actually hit first and who said what beforehand in a world without cellphone cameras, which was the case when our legal code was written. The courts were not interested in trying to sort out the roughly 100% of cases where both sides claim to have hit second in self defense after doing nothing wrong at all, officer. You're both guilty.

That said, this one is on camera and that probably works against mister sucker punch here, since it's a fundamental principal of US law that your recourse if someone commits a crime against you is to go through the court system, not to wrong them back.

I'm not gonna critique the practicality or actual fairness of that, it's just how things are here.

Caveats: 50 states have 50 different legal codes with 50 different sets of exceptions, and also I'm not a lawyer just a dude who took an intro criminology course for a gen ed in college a long time ago.