I don't think that Ubuntu claims to be fully GPL, or even fully Open Source.
For that, you need to use a variant called Gobuntu:
www.ubuntu.com/products/whatisubuntu/gobuntu
Now, when Apple built OS X, they based it on the Berkeley Software
Distribution (BSD) and later, certain parts from FreeBSD's and
NetBSD's implementation of Unix in Nextstep (being the OS Jobs
developed for his NeXT machines, after he left Apple in 1985, or
thereabouts). Mac OS X was originally written for use on the old
PowerPC hardware, rather than Intel, and would have likely been
rewritten and optimised to make it run more efficiently ... then,
eventually ported. Sounds like they've released a whole bunch of this
stuff into the wild, hence the copyright legend.
If you look hard enough within the OS (Linux), you may find copyrights
beloning to a whole host of companies, such as the University of
California at Berkley AT&T Bell Laboratories, Sun Microsystems,
Hewlett Packard, IBM and Carnegie Mellon (amongst others) - you'll
find these in most Unix OS variants, being the historical developers
and maintainers of Unix.
I think that Microsoft are trying to claim copyright/ownership due to
the fact that they originally developed a Unix variant OS some years
ago, called Xenix. BG wasn't happy about having to pay royalties to
AT&T, et al, for every copy he sold, so sold it on to somebody called
the Santa Cruz Operation, where it became developed into SCO Unix.
Also one of the main reasons that Microsoft were so keen to throw a
lot of funding behind SCO's recent ownership claims ...