It is painful in how obviously little you know about how a gamestop works. Just because you have experience with perishable goods does not mean you know how their business works.
They can do whatever they want without worrying about alienating the publishers. Gamestop and the Publishers both know the amount of people that shop at those god forsaken stores. Even if they piss off the publishers the publishers arent likely to pull their products because they know that people do buy there still in large numbers, and even with the damage gamestop and their usedgame groupies do, they would be hurting themselves more by not letting gamestop carry their stuff. It is also important to know you can trade in a game you bought anywhere to gamestop, so if say EA doesnt let gamestop carry their crap, someone could still buy an EA game at Walmart then trade it in to gamestop for them to resell.
The largest amount of damage used games do are to the tail end of a product, where companies could rely on a longer life of their product by people buying new through weeks and months as price goes gradually down, now people buy it new, beat their games then trade them in, then gamestop pushes these used games, so the games no longer are being bought new a few weeks after they are released.
And the subtle pressure pisses me off about gamestop. "Oh you know we have used copies of this game right?" "Yeah, I want it new." "Why dont ya like saving money? Here, I can go get you one off the shelf for you." "No, just give me my damn game." "Oh, alright, but hey, if you bring this game back within a week we will give you 15 dollars for it, after that you will get only 10." (Actual conversation)