Assuming this is actually TRUE, it's not a good move.
While informed harcore gamers know what they want when they go to buy a game, the less hardcore crowd don't and games with blocked covers will be ignored over ones that can be seen, resulting in lost sales. That's something I don't think Wal~Mart wants.
M games are already restricted at the register(prompts for an ID check) so unless the cashier isn't doing their job, the ball is in the parent's court, not the retailer's.