Well, not really - the Darwin kernel is Open Source, etc. and is heavily used by the Hackintosh community, for example. Apple keeps publishing the xnu sources along with the rest (including all the other APSL licensed components in the system) for each OS update.
The non-free parts of Mac OS X are way up the software stack, where Apple's got a unique selling point. The kernel isn't, really - it would be entirely possible for Apple to do the same thing on top of Linux (like Google does right now for Android, where pretty much everything on top of the GPLd Linux kernel is kept proprietary & locked down), if they cared about hacking on the Linux kernel.
Apple doesn't, though, as it's much easier for them to hack on a kernel they have control over - less communication & negotiation overhead.