Unix is basically a brand name.
BSD had to be completely re-written to remove all Unix code, so it could be published under a free license.
It isn't Unix certified.
So it is Unix-derived, but not currently a Unix system (which is a completely meaningless term anyway).
It means nothing, it's just a paycheck you sign and then you get to say "I certify my OS is Unix". The little bit more technical part is POSIX compliance but modern OSs are such massive and complex beasts today that those compliances are tiny parts and very slowly but very surely becoming irrelevant over time.
Apple made OSX Unix certified because it was cheap and it got them off the hook from a lawsuit. That's it.
Yup. It is all about paying the price, Microsoft could technically get Windows certified as UNIX. IBM did just that with its mainframe OS. Here's a list of certified UNIX systems: https://www.opengroup.org/openbrand/register/
Microsoft could technically get Windows certified as UNIX.
I don't think they could now that the POSIX subsystem and Windows Services for UNIX are both gone. Don't you need at least some level of POSIX compliance (at least the parts where POSIX and Unix standards overlap) to get Unix certified?
To make it more specific I guess, what's the problem with that? It's like having a "people living on boats" and "people with no long term address". You could include the former in the latter, but then you are just conveying less information.
Others have answered, but it is interesting to know the history of UNIX and why this came to be. BSD is technically UNIX derived, but being more specific isn't the reason why it has distinct branding. As with many evils the root is money, and there's a lot in play into how it all happened, including AT&T being a phone monopoly.