I think there is a difference, though: I'd imagine Walmart would buy a certain amount of copies of your game, and they would negotiate their purchase price, squeezing you as much as they can. Then they would mark it up and sell it on their own. If they see that you are selling it for half the price on your website, then that gives them a stronger purchasing argument to lower your price.

Apple, on the other hand, is imposing restrictions on which price you must sell it to third parties, and, that, I think, is illegal.