I think it's good for everyone that we openly talk about Lion. It allows developers to figure out how features work, why they were added / changed / removed, etc... I don't think this is bad for Apple in any way. The most ideal situation would be that everyone would be able to test Lion and discuss it. That would give enough feedback for Apple, if they had infinite resources to read all of the feedback (remember, in an ideal world).
Also, this generates way more hype. You can't keep this stuff secret. Anyone can join the developer program, does Apple really expect every single "developer" to keep quiet about Lion? Of course not. They know exactly that this beta is quite public. They don't mind. As long as people file bug reports and not just post about the bugs, it's fine.
There's an NDA because Apple wants to be able to control things if something gets out of hand (press, media, copyright, I don't know).