You are overreacting. He followed his policy. What's wrong with that? Even if he did have "an attitude" who gives a forget?
You extrememly overreacted
they are LEGALLY required to say that. the Game Rating Administration makes them.
You are overreacting.
He was doing his job.
Just admit you are wrong. 
No, he's legally required to warn me about the content. Not suggest that I put the game back on the shelf and tell my brother he's not mature enough for the game.
By the way, I just said, "I'm aware." I bought the game for him. But that cashier was a richard.