That is only a thing on i(Pad)OS. There's nothing to my knowledge preventing anyone from running their own app store on macOS - Steam purchases work fine, the entire Adobe suite does, as do Macports and Homebrew.
But Apple has proved that they will enforce their walled garden if and when they feel like it. Any company building a thriving profitable ecosystem inside Apple's walled garden is at risk of the owner of the garden starting to tax them.
It's a little like licensing Enterprise software from Oracle. You know they'll be looking for reasons to you sue you. You may be able to avoid it, but you know it's in their style.
