That's just ignorance. Of course there are rules. At the very minimum, there are laws. If you don't act (and train) with respect to the laws there can be some pretty dire consequences. Unless you like community showers, no privacy and spending time in large crowds of people who are generally either asocial or antisocial.
That's without including the local taboos.
Oh, yeah, there are rules in a street fight.
But there is no such thing as cheating. That's a subtle distinction.
Rules and cheating are social ideas, things designed to keep you at a very specific level of interaction. If you cheat as a child playing games, you won't have any friends. You cheat at a card game and you may lose more than friends, depending on the culture. You might get knifed or you might get voted out of the country club.
Rules keep everything hunky dory in the tribe. The big rules are physics, the big social rules are laws. The rest are just agreements and expectations. Most unwritten, most things we just do, subconsciously, because we have always seen them done. Alternatives don't occur to us. We could just move our little Monopoly doggy to 'Go' every time, regardless of what the dice say. But we don't. That would be cheating. And no one told us that. That is what we, as socialized individuals, bring to the table.
There is no such thing as cheating when you are under attack. You're a good person. You don't cheat. And so you hesitate, not doing things you know would work because you aren't sure if you will violate social taboos. If it's going to violence, guess what? The social taboos have already been pretty much nullified. There are rules in a fight. Please don't go to prison. But there aren't a lot of rules unless you bring them in your own head. If you do, the rules in your head only apply to you.
There is nothing you can do under assault that will make the other kids say, "I don't want to play with you any more! You're a big cheater!"
And you know what? If their idea of play involves a criminal assault, I'm okay with it if they don't want to play with me anymore.