They are not

"Steam keys are meant to be a convenient tool for game developers to sell their game on other stores and at retail. Steam keys are free and can be activated by customers on Steam to grant a license to a product."

https://partner.steamgames.com/doc/features/keys

I didn’t mean to say they are getting a direct cut as sibling commenters indicated, but Valve definitely is getting at least some kind of money indirectly out of them offering this.

I'm a developer who uses Steamworks. Keys are free for us to generate and sell at will. Valve reserve the right to say no, but from what I can tell, their only policy is to not take the piss (selling millions of keys), and not to charge more to customers who buy directly from Steam.