There's a huge difference between GPS which is effectively a rootkit, and Steam which is a userland application however.

For now. But also, phone companies can use Android without GPS, but they have to rebuild a lot of functionally to get it to work, and this was done as a design decision by Google to maintain control over Android. I can easily see Valve doing this in the future. Hell, imagine Valve buying Unity and integrating it to Steam while keeping developer costs low.