Lukaszmik: What could they do? How about a contractual clause in any publishing agreement forcing the developer to maintain such parity? Shouldn't be THAT difficult to add it to current terms, and if there are any financial penalties and/or threat of being de-listed coupled with it, that should be enough to make any studio think twice before pulling this kind of bullshit.
And if somebody wants to use GOG for cash-and-run release, as you said - fuck 'em.
As tfishell pointed out, gog is in no position to strongarm anyone.
Yes, they could put clauses into their contracts (they probably do already), but *enforcing* these clauses is an entirely different matter. Sure, gog could tell them "Here's the door!", they'd just go "Fine, we'll sell exclusively on Steam then!". They'd be loosing a couple of hundred bucks, if that.
I'm afraid the main way for gog out of this mess is to stop selling new games, become Good Old Games again and only sell games that are sure not to receive any more patches. And even then you might still get screwed over royally (see Cinemaware).
Unfortunately, it's up to the customer to decide whether or not they want to run the risk of buying a game here. And it's true, this issue is damaging to gog's reputation (and I'm already seeing myself buying fewer and fewer games here - especially when it comes to new games). The fact that, for example, something like Perception is still being sold is mind-boggling.