If you look at the forum index, only the ones under "Full Games" (i.e. The Babylon Project, Wing Commander Saga and Wings of Dawn) and Diaspora do not require FreeSpace 2 itself. Everything else, including BP, needs a copy of FS2.
It's true that the source code was release for free, but in effect that only means that modders have more ability to change things, it doesn't mean you get the actual game for free. As I understood it, all the models, textures, mission, soundfiles and stuff are still protected by the IP and copyright laws and thus the entire game can't be given out for free unless it's a total conversion that replaces all (or at least the vast majority) of those assets, like the aforementioned projects.