Microsoft cut their margins because they didn't want to get sued.
A company should be free to sell its game in any way at any price without any restriction coming from one vendor.
And that's exactly what Steam's agreement is. If you sell on Steam, you can sell your game with or without Steam keys on your own website, you can sell on any competitor's platform, and you can cancel your game from Steam at any time. There's no lock in here. You can even add your own DRM or no DRM at all (or use theirs), you can make your game free and only sell additional content through your own website (where you keep all profit), etc. There's no lock in whatsoever.