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They did, but Apple threatened them, and it was removed from popular theme sites, but I'm sure you can still find it.
What a load of codswallop - GNOME and KDE theme sites till host Aqua inspired themes for their respective desktops.
Apple got in a tizzy by the fact that people were copying native icons and making them available free of charge for other platforms, and also including the use of trademarks such as the Apple logo and so forth - I see nothing wrong in what Apple did; its no different to, say, Adobe turning around suing people for copying fonts out of their Studio CS and making them available for download free of charge.
Oh, and back to the issue; they don't need to bundle the icons with the source distribution; it would simply be a matter of setting up some macros in the configure script so that when it detects MacOS X, it uses the icons that are native on the machine, thus not requiring any copying.