h.e.X.e.n.: but they have the serial number\ key of my copy right?
Wrong. Your installer is the same installer that everyone on GOG gets. Nobody could idetify "your" installer. Not having shit attached to your games is exactly what DRM free means.
Pheace: Wasn't the whole point of GOG's DRM-Free stance to not treat customers as criminals though? Is this really any different?
Though I imagine the problem here would be that it's too hard to sell to the publishers/devs. Exactly this. I don't think that anyone would go through the hassle to buy a game, download it, ask for a refund and to play his ilegal (your installer becomes as legal as any pirated installer) and (if it's a newer game) unpatched game. If someone wants an illegal copy, s/he'll just pirate it. "Buyrating" it from GOG is just too complicated.