When you buy something, it’s your job as a consumer to inspect the item prior to purchase. If the item has been opened and there is evidence of it or a notice has been posted then you have no legal leg to stand on. However, if Gamestop is re-sealing the packages in a manner that leaves the average consumer unaware that the item has been tamperedwith and doesn’t inform the customer in any way then yes it most certainly IS illegal and they can at the very least be sued over it. I wouldn’t be surprised if someone couldn’t be arrested for ordering employees to do it. That’s so sketchy as hell there has to be a law against it.
That’s like you buying a deluxe or gold edition of a game and getting home to discover that the bundled expansions were removed by the retailer. You bought the game as a whole, not the game as the retailer thinks it should be sold.