bbPress is a standalone product, similar to WordPress MU and WordPress.org. The idea is to keep everything modular, and allow people to pick and choose what they want to use.
I think that the way that the Forums are implemented into BuddyPress will change once BackPress because a more mature API.
In the meantime, the forums work almost exactly like the wire does, but for groups instead. Groups also have their own wire, so in reality having a forum is a little redundant until someone takes the initiative to introduce some more functionality into the BuddyPress forum area.