OS X is not "far more closed". Darwin, the UNIX kernel and utils that OS X runs on top of is fully free and open-source. It's just the Apple-specific APIs like Cocoa, Carbon, Quartz, Aqua and all that jazz that is closed-source and proprietary.
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.
Good luck building them. Many of the code dumps are incomplete (like dyld and ld64). They reference private apple headers and won't build without them. Granted, they can probably be hacked to build in some cases and having the source just for reading is still nice.