The aren't contributing to Linux to be nice. The entire reason they started it was because Microsoft started selling games in the Microsoft store and they were afraid of competition from the OS vendor.
It's why they rushed the first generation of Steam Machines to market way before they were ready.
The only thing that keeps Steam from being as awful as other services is that they're not a public company, meaning they take a longer-term view instead of the quarterly pump and dump. But their singular goal with Steam is and always has been to make unholy amounts of money.