Yup, NDA signings keep people from blabbing.
That said, I'd love to have a "Director's Commentary" thing for books. Like, a year after it was published, a thing where the writers could post thing sto the forums, and go, "Okay, this thing here was a reference to this plot. This thing was a plotline for any GM to pick up on. This guy here was named after my cousin, while this other one was a girl I used to date. Oh! This part! Yeah, I love dthis part! I'd been doing a lot of reading about Thing the past month, and this happened to come up *right* when I was filled with useful info. Perfect timing!"
Stuff like that.
Until it hits shelves, though, saying anything can get pink slips, and nobody wants that. Heck, even when it hits shelves, there's some stuff that we shouldn't say. "Oh good, Awesome Book is out? Check out page blankity-blank! That's where I kick off this AWESOME plot that'll be running for the next three years. Cool huh?" D'oh. Fired.
The older something is, the safer it is to talk about. As time goes by and I learn more about what's going on, you might notice that my speculation has slowed down. Pretty soon, I won't be able to do it at all because, well, I chat people up all the time to try and stay current. Mixing up what's being written and what's been released is a quick way to blurt something and get your head whapped.
And I have a tender skull.