Why did GNU choose UNIX?

Or rather, why did rms et al dislike UNIX? It seems to come up a lot that they basically settled for a UNIX clone since it was widespread. They certainly didn't care for the philosophy.

I never used Unix (not even for a minute) until after I decided to develop a free replacement for it (the GNU system). I chose that design to follow because it was portable and seemed fairly clean. I was never a fan of Unix; I had some criticisms of it too. But it was ok overall as a model.
- http://stallman.org/stallman-computing.html

Prior to GNU, Stallman was working on a proprietary OS being developed by MIT, which I can imagine was probably superior to Unix in some ways.

I personally use Unix-likes because they're better than Windows, not because they're perfect.