The comment I was replying to said "So does every app maker" rather than "platform owner".
[I also added "(hardware)" to my original comment.]
Yeah that was my bad. I meant platform owners
Don't write an angry reply before your morning coffee everyone!
Steam takes a cut of the sales price and a cut of microtransactions done through the Steam wallet. But they don't force developers to use the Steam wallet. I wouldn't be surprised if that last point is what will ultimately matter.