They’re an http download website that takes a 30% cut on the hard work of artists.
That is already reprehensible, but not illegal.
They also dominate their market and their competition is laughable.
Now that is a properly punishable crime.
Developers can and almost always do close to offer their games on multiple platforms and can even choose self hosted direct distribution of they do choose. Customers can choose to purchase their games on steam, itch, epic, Microsoft, or any of the many places they’re often hosted simultaneously. Steam is more often than not the choice people choose to use of their own free will because they perceive it as being the superior service.
Why do you believe excellence should be punished?