I never heard of a store, especially WalMart, setting their own standards for games over the ESRB rating. That's really odd. I think it's ridiculous that they would want you to be 17 for this game.
That happens all the time, stores set their own policies when it comes to that sort of thing. Here where I live there was a store that made no exceptions to the age limit, so there I was at 51 having to show my ID in order to prove I was old enough to play a game. They later eased up on that, no doubt due to the many complaints they were getting. I certainly spoke my mind about the inconvenience and absurdity of it. That's just political correctness gone wild.