Facebook claims here,
The patent grant says that if you're going to use the software we've released under it, you lose the patent license from us if you sue us for patent infringement. We believe that if this license were widely adopted, it could actually reduce meritless litigation for all adopters, and we want to work with others to explore this possibility.
Does Facebook have any reason to believe that? Have they ever been a defendant in a patent suit by a company that used products that they released under the BSD license? It seems like they're trying to turn their software into a legal weapon.
I sense drama here, and I just wanted to know more.