I assume you barely ever shop there or I misunderstood your message. But they always swap the display copy out for a sealed one, unless the display copy is the last one available. It's not a new policy, at least up here in upstate NY. It's been standard operating procedure since the ones up here were Electronics Boutique years ago.
Are you saying the display case wasn't a gutted new game? I've only seen that when they wanted a physical game case on the shelf for preorders of big games.