Steam
is a closed platform, but:
It's easier to beat Valve's lawyers than Sony's. Sony has a long history of paying out the nose for lawyers who could convince a judge that a law saying "Kill no one" meant "Genocide is legal for Sony Corporation!" (See: https://en.wikipedia.org/wiki/Sony_Computer_Entertainment_Am...)
Apple and Microsoft are the two most valuable companies on the planet right now, and they, too, can get some great lawyers (though they haven't demonstrated the same courtroom magic that Sony has).
I can't remember who originally was responsible for this quote, but "Nintendo is a pile of cash disguised as a video game company" is something that has been proven anything but incorrect over the past decades. Wouldn't want to pick a fight in any court with them, either. (Relevant: https://en.wikipedia.org/wiki/History_of_Nintendo)
Valve, though? Valve is worth little in comparison, and hasn't had a great history in court. Easy pickings. Then, after you win against Valve, you go up, using the precedent set to help you with the next target.