It has been discussed, there's just no reason to bring it up frequently. All of Baen Free Library is just that, free, so there's no reason to keep everyone up to date with what's available from them for free.
If you search, though, you will find that Baen has been discussed in various threads for some time.
While Baen has been discussed before, IMO it is always good to repost periodically. New forum members would not know to search for Baen threads. This would be like saying there is no reason to update the free book list since people could search for the book themselves.
It's a gray area. The user guidelines read "Direct links to other commercial websites are, in the main, inappropriate for our Book Club conversations." Generally in the past, the way that has been inforced has implied it means "no direct linking to product pages from other book sellers when B&N also sells the book, and no using these boards as an advertising forum for your off-site business."
So, linking to Baen Free Library may be technically against the user guidelines (it is a commercial website, in the sense that it's a website run by a business for commercial purposes), but that's not how the guideline is currently being applied, nor do I think it's the way they meant it to be applied when the crafted the guidelines.
Personally, I wish that particular guideline was a little bit clearer - for example, is linking to goodreads, a social cataloguing book website, kosher? What about inkmesh, an ebook shopping search engine? What about simply linking to Baen's web distributor webscriptions, without actually linking to any particular book there? Linking to Wikipedia, which is partially advertising-supported? And so forth, and so on.
If I were simply trying to take that one line in it's immediate context, all it would be banning is advertising on the board, but the moderators have gone somewhat farther (censoring links that were clearly posted in response to questions or requests, for informational purposes, rather than to advertise an alternate website).
Hey, you seem knowledgeable about the baen free library so I'm hoping you can enlighten someone who just discovered it. How do I transfer books from the library onto my nook (if that's even possible)