Why is one only considered protection?
Brass knuckles are generally considered to be an aggressive weapon. While you can intimidate a person with a gun and a gun is a great equalizer for people with less strength or physical stature than their attacker, with brass knuckles you basically have to be able to beat the shit out of someone bare handed already and really the only way to use brass knuckles effectively is to beat the shit out of someone with them. That being said, I don’t think banning brass knuckles makes sense and it’s a stupid thing to do, but the mindset of carrying brass knuckles is thought to be very different therefor politicians look less favorably on them. In my state you can carry both though so none of this really applies here.
I think there’s a state or two in which the carry license is a weapons license and theoretically brass knuckles would be a go, but I’m not sure.
But yes, wearing brass knuckles messes up the grappling so it’s just for punching. I still think it would make a good defense tool. It’s an inanimate metal object, it shouldn’t be illegal to carry.
Maybe its some kind of “handkerchief law” due to street thugs using brass knuckled, if you stopped some guy from the mafia and couldn’t pin a serious crime on him, you could charge him with carrying a deadly weapon.