Well, the implicit assumption behind FB's bsd+patents license is that you should be ok with anyone "infringing" on your software patents, since software patents are mostly bullshit and you only acquired them to be used in defense (i. e. never suing someone first). That license doesn't make any sense if you believe otherwise.
[1] The language I'm thinking of is the part that says that the license terminates if you initiate "any Patent Assertion: (i) against Facebook or any of its subsidiaries or corporate affiliates, (ii) . . . "
They probably left it this way because there is no legal definition of a "software patent" - they cannot differentiate between those and "other patents". It's not like "other patents" are all sunshine and rainbows either - the patent system is fundamentally broken.