Our games have a security case on them, that they can open at the register. If no one removes it, it will trigger an alarm when you leave.
Are people in the US really bringing tools to the store to break them inside the store? And so frequently that it becomes a problem?
As for limited selection of games compared to Gamestop, that's probably the case here as well.
I've never heard of it, but loss prevention is a big thing with Walmart with the volume of customers they have. That's how they've pretty much always been. There are stores here that use alarm boxes.
As for new games, the big retailers generally limit their selection to the big name games from EA, Activision, Ubisoft, Take-Two, WB, or the console makers. You'll find Final Fantasy XV pretty readily at Walmart, but Dragon Quest XI is a bit more iffy, and good luck finding a Walmart that will carry something like SNK 40th Anniversary Collection.