Remember, Amazon managed to strong arm i-Tunes and the music industry into DRM free mp3s. Amazon does leverage the fact that they're a fucking massive retail outlet as well, these publishers aren't dealing with a digital only service, but a Walmart sized retail organization that spans the globe. I'm not so sure that the publishers get to dictate much at all to Amazon.
Well, something will have to give at some point. Either something's going to go wrong between Publishers and Amazon, or when half the world is shopping at Amazon for $ prices, something will give between the publishers and the rest of the retailers. I'm really not prone to believe the latter one will actually become a reality to be honest.
Also, it's not like this is some stance Amazon is making at the moment. They already stated there are legal reasons they have those requirements for purchases. In the best case right now, they are condoning, and not enforcing the actual accuracy of the purchasing details they are being given. That's not exactly an idealistic stance or anything, that's just playing with the rules for their own benefit.
Granted, the best profits from that tactic are probably still the titles that actually sell Steam Keys as they seem to be the most wanted by far, which kind of leaves in the middle whether this is actually a good or a bad thing for Steam in the long run.