...How? Are they hiding the game under their shirt until you pay or something
You bring the game up in a sealed copy then they put it in their drawer and just start putting a sleeved disc into an open case without mentioning it. Unless you are questioning what they are doing, they just do it. And, some casuals don't even know the difference.
...Well, first, I'd wonder what the hell kind of Gamestop puts sealed copies on the shelf.