Exactly! Pirate Software talked about this a while back. Steam doesn't want you cutting them out, and then them still being responsible for the bandwidth to download and host your game.

I am almost certain that steam keys are actually free to developers, which is the whole reason for the policy.

Yes, they are. That's what many of the Kinguin etc. keys are. People/bots pretend to be game reviewers/streamers and ask for free keys. I have a "Game Press" license for a game because back then I didn't know of that method. I was under the impression those were keys sold by the developer in foreign markets for adjusted prices. Now I know better.