That's pretty messed up. That's never happened with my GameStop, though. Those guys are awesome. They actually know what the hell they're talking about (as opposed to the two GameStops up the street), and they have their own policies that they enforce on the employees. For example, that "borrowing" action you just talked about is allowed, but once they return it they have to label it as being pre-played. The employee pays part of what full game costs, and they sell it at a marked-down price. They also use their borrowing to let people play the game at midnight launches before it comes out. Ours broke open a copy of Batman: Arkham Asylum two hours before the midnight launch, so people could try it out. None of the other GameStops nearby were doing that.
Also, I've seen through a doorway what their back rooms look like, and they have a fridge where someone scribbled a Triforce and wrote "Ice Temple".

The thing is, they were busted earlier this year taking those pre-release copies back from employees and then NOT marking them as pre-played, but selling them as brand new. That doesn't mean every store did it, but it was a high enough majority that it seemed to be a company policy.