Valve does allow listing on other stores at other prices. They even allow you to sell Steam keys for your game on other stores for lower prices - and they earn $0 for that while still providing the infrastructure for those players to download/play.
Edit: I think for steam key sales actually you're supposed to sell for the same price as Steam, but I can't seem to find the documentatiom stating either way
Edit 2: Ah, here is something stating it "You should use Steam Keys to sell your game on other stores in a similar way to how you sell your game on Steam. It is important that you don’t give Steam customers a worse deal than Steam Key purchasers." (https://partner.steamgames.com/doc/features/keys)
However, I know many games have steam keys indeed available for cheaper on other stores. Not sure how much valve actually cares, or if they have special permission or what.
Edit: I think for steam key sales actually you're supposed to sell for the same price as Steam, but I can't seem to find the documentatiom stating either way
Edit 2: Ah, here is something stating it "You should use Steam Keys to sell your game on other stores in a similar way to how you sell your game on Steam. It is important that you don’t give Steam customers a worse deal than Steam Key purchasers." (https://partner.steamgames.com/doc/features/keys)
However, I know many games have steam keys indeed available for cheaper on other stores. Not sure how much valve actually cares, or if they have special permission or what.