I'm not comparing the game to alcohol, just same type of rules apply there. 

If the guy wanted to buy the game he should have had his brother go somewhere else.  If he just didn't think of that, then she should have just said OK, left the store, come back alone a few minutes later, and then got the game.

 It's not illegal for someone under 17 to play an M rated game and it's actually not illegal for someone under 21 to drink, but it illegal for someone under 17 to buy an M rated game and someone under 21 to buy alcohol.  

They are different things but it's still the same situation: if you're buying the item in question  (M rated game, alcohol, whatever) and you have someone under the age whom you're not a guardian of, the seller has every right and should assume you're buying it for them.

But legally he's allowed to buy the game. He could have had an entire kindergarten class with him and he should be allowed to buy the game. He's over 17 and that's that.

Compare this to an adult bringing a minor into an R rated movie. It's perfectly fine and happens all the time.  

 Like I said, with the trail of kindergarteners behind him the seller can assume that they are buying it for them.  Like I said: good for the cashier-- I would much rather see this story than one about some kid who maliciously got a hold of an M rated game.

I meant it's illegal to buy alcohol for a minor. You're not supposed to do that. A parent can't buy alcohol for their kids, they would get arressted.

It's NOT Illegal to buy an M rated game for a minor.  Minors are allowed to buy M-rated games as long as they have someone over 17 with them. This one did(and he wasn't even the one buying the game)

The rules are completely different. 

 You're almost right: someone under 17 is allowed to buy an M rated game not if someone over 17 is with them, if their legal guardian who is over 17 with them.  There's a difference.

The brother wasn't the kids legal guardian so no game. 

twesterm is absolutely right. If the older person in question was his legal guardian then that's fine, but the cashier did their job. She was completely right to refuse sale to a kid who wasn't accompanied by his legal guardian.

I guess it depends on what state you live in. My state has the strictest game laws and I work in the game store. Anyone over 17 is allowed to buy M rated games regardless of who is with them. 

Also I'd like to know how Bestbuy determines who the legal guaridan of someone is...That's not a rule that anyone enforces. I shouldn't have to bring legal documentation with me to Bestbuy when I want to buy a game. That's completely foolish and going way overboard.

The bottom line is that the guy had a giftcard, he's legally allowed to buy the game and they didn't sell it to him. The Bestbuy employee can make whatever assumptions they want but they can't act on it.

Imagine if I started refusing costumers as my store cause I thought they were buying games for kids. You also can't get your money back with the giftcards, you have to buy something in the store or it's wasted, he tried to do that and they refused him based on the assumption of an employee.  

 She didn't refuse the kid,she refused the adult just because he was with a minor. 

I have never heard of a law that states in order to purchase alcohol, cigarettes, or in this case, a video game, then the adult has to be a legal guardian if they go to the store with a minor. What if it was an uncle or an aunt, would to same argument hold water?

I think the main problem was that he took those gift cards from the younger brother.