Well it kind of is. A modified BSD kernel (open source, called darwin) that can't run without heavy modifications on non-apple hardware, with lots of proprietar

Well it kind of is. A modified BSD kernel (open source, called darwin) that can't run without heavy modifications on non-apple hardware, with lots of proprietary libraries bolted in. So they effectively took an open source software and made it hard to use outside their own hardware. So unless I'm missing something, macOS is basically a "tivoized" BSD. I agree though that NeXT didn't "steal BSD". BSD is liberally licensed, so NeXT was perfectly within their rights to use it. Legally speaking, I don't think they even needed to open source their fork.