The game is NOT restricted to adults. It is rated Mature.
Parents have every right (at least in America) to buy the game for their kid if they so choose. Whether that is a good idea or not is another debate. But you have no right to tell someone what they can or can't buy for their kid.
It is completely irrelevant if you or even I feel differently. This is America baby!
Actually, in America, it's currently illegal to buy a 'mature' rated game for the implicit reason of providing it to a minor and has been since 2005.
However it is not illegal to buy a game for yourself and allow a minor to use it.
It actually does not matter if the person buying the game for a minor is there parent, family or a creepy guy at the bottom of the street. The law sees no difference in the provision of content to minors. It's just nobody has been taken to court yet for a rule to be made in law.
It's America, baby, the most eloquent lawyer makes the law. All it needs is a decent public prosecutor, and a cheap ass defensive lawyer, and it will be a test case enshrined in law. Just wait for a state DA to want to make a political point.