I agree: what they are doing is understandable, and defensible imho.
The foundation of OS/X is Darwin. The code is
built using derivatives of the GNU compiler-suite. But the entire system, as a whole, is not ... and has never been.
I should note also that Apple has
contributed some of the stuff which they developed .. which they
paid to develop .. and they furnish it today in source-code form.
Like every company, Apple's in business to make money, and they make money by producing
a tightly-designed mesh of hardware and software, both designed for one another. They use an open-source foundation, and they use open-source tools, but they
sell the final product. I do not recall at the moment if they offer any (closed) source-code licenses to any segments. "Their product" is "a computer system."
If you look closely, you'll notice that there are Linux-based vendors out there doing exactly the same thing. It's how they pay their developers
salaries.