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Can I get source access to the NT kernel on Windows? Nope. That's Microsoft's secret and novel invention that is supposedly the best OS in the world, so they have to keep it on lockdown. </s>
Just being restrictive of what computers can run your OS does not make it closed. When has Microha$h ever supported FOSS? Think about it for a second.
OS X is specifically designed not to work non-Apple hardware, even hardware that is perfectly capable of running it.
Parts of it are open source, mostly because those parts were an extention of existing open source software.
"OSX" is not free in any sense of the word.
Microsoft are still a far cry from the likes of GNU/Linux in terms of openness, but almost nobody would describe the Mac platform as "more open" than Windows.
In Windows: "Metro" apps didn't exist before Windows 8, and normal apps don't have any origin restrictions.
In OSX: Gatekeeper is easily bypassed by the user for specific apps or even turned off entirely.
Which means Steam and all its titles would have to be launched by explicitly navigating to the classic desktop. Which is beyond Apple's simply providing a parallel way to get software onto OSX.
If Apple didn't allow icons in the Dock, unless the app came from the Mac App Store, I'm sure you'd see a similar reaction from third-party platform creators like Valve.
