That is a given. When minors see how M rated this game can be, they'll still get it. They'll convince their parents that Batman is a relatable, "kid-friendly" character.
We are talking about the USA here. Excessive violence is of no concern for the vast majority of parents, so kids won't have to do any convincing.
Besides, ratings for video games are merely guidelines, not legally binding like the sale of alcohol. Most clerks aren't going to care if a kid buys an M-rated game on their own.