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.
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.