I have to wonder, why does Apple even make Darwin open source?
It doesn’t seem like it’s of much use to Apple—there isn't some large community of outside contributors submitting pull requests. So why do they bother?
One time I used Darwin open source to make a printf-sprinkled version of the dynamic linker to track down a bug that was causing our app to not start on about 1/3 of our install base. There were no error messages or console logs and the logs produced by feeding dyld environment variables were not helpful, so the ability to build a custom dyld was instrumental.
The problem wound up being that we leaked a sticky bit directory into part of our install, the installer inspector hid this from us, and apple changed dyld's behavior in a patch release to silently bail if it saw a directory with sticky bit set.
Licence compliance -- they took the Unix kernel and don't really want to give back but having now taken the kernel it now has obligations under that licence to make it available for others.
Which license? I'm not aware of any obligations to release Darwin's source on Apple's part—hence how they're able to wait half a year before releasing the source of new versions.
It doesn’t seem like it’s of much use to Apple—there isn't some large community of outside contributors submitting pull requests. So why do they bother?