It seems that from 10.5 this caveat was added to the official OS X documentation. In that light I think it's safest to conclude that Apple realised fork() is hard (check out the long list of things a current Linux fork does to retain sanity in the face of threads, asynchronous I/O, capabilities and other fun toys that didn't exist at the dawn of Unix) and decided they don't care. It will probably work, but if it doesn't they aren't interested in explaining why/ fixing the problem.
On balance I agree this makes OS X a pretty shoddy Unix, but then, I would have been easily persuaded of that anyway.
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