Updates since 2004? It's like they took an old snapshot of BSD userspace and some of the syscall interface, except somewhat broken (see the long saga of broken select/kqueue in OSX). Fundamentally Darwin is a completely different beast based on Mach. It's radically different from the core BSD kernel. Most of their userspace is built on Objective-C, which has 0 overlap with BSDs.
It's cool that the Mac community used some BSD code way back when — that's why we use a permissive license — but they've forked whatever they took; they never work to integrate their changes back upstream. To the extent that they're a BSD, they're a very very divergent fork of a historical BSD. And that's fine, for them. I don't know if that makes them a BSD or not in some ontological sense.
To be clear, they mostly publish their changes under open or even copyleft (APSL) licenses. The problem isn't one that the GPL solves. They just dump the modified sources on their FTP and make no attempt to interact with upstream.
The limited exception is Clang/LLVM, where to some extent they are the upstream.
(A clarification: I also like the MIT license, but the majority of BSD is published under the so-named BSD license, with some number of other clauses. You might already know that, and if so, sorry for repeating the explanation; I wasn't sure from your comment. They're both permissive, but not identical licenses.)
It's cool that the Mac community used some BSD code way back when — that's why we use a permissive license — but they've forked whatever they took; they never work to integrate their changes back upstream. To the extent that they're a BSD, they're a very very divergent fork of a historical BSD. And that's fine, for them. I don't know if that makes them a BSD or not in some ontological sense.