It just makes sense from their point of view and also to some others out there who are looking at it objectively.
Guess who's point of view I have!
Seriously; the moaning about (and now charging for) the second hand games market like it's some sort of non-legit activity is a pretty big anti-consumer move on Sony's (and other publisher's) part.
IANAL either, but I would expect that GameStop owns the games until they sell them, and can do with them as they wish, given that the games are their property. What they are obligated to do or not to do contractually with the game maker may be a different story.
Would messing with the contents of the game box affect their ability to claim that it's "new"? I genuinely don't know but my expectation of buying a new product would be that it is the product the manufacturer shipped out, no changes, no wear and tear (well, transport bumps), no missing pieces etc. Taking stuff out the box and selling as new is dodgy in my opinion but I don't know the legal stuff here.
What is research but a blind date with knowledge? -- Will Harvey