I never understood this either. I kind of don't like the idea of buying a book that who knows how many other people have already read while in the store. To me, that's buying a "used" book.
I get the feeling that B&N doesn't actually encourage this -- that the chairs are there for you to maybe browse through a few books to see if they're what you're looking for, but if someone spends a few hours there reading an entire book, what are they going to say about it? Kind of like they tolerate it, but don't really encourage it. What would be the point of having books for sale?
I would feel funny doing it, personally....