Without free software, there’s no free market. In the Microsoft/Apple ecologies, there is only the PERCEPTION of a free market. It isn’t truly free to anyone if Microsoft/Apple can come around and change or impose the rules whenever they feel like it, regardless of who’s work suffers.
In the free software ecology, there is plenty of freedom for developers. Alas, freedom is not free, and that is why there are a seemingly daunting amount of graphical user interfaces, APIs, licenses, and so on. The bottom line is; man up, Valve, make your own way. Free platforms even welcome closed source works, you know, like Steam.