I agree with you, but I understand the issue being raised. The way I see this, the issue breaks down three ways (not chronologically):
1) Facebook publishes open source software, that everyone is free to use under copyright law. Everyone agrees this is a good thing.
2) The copyrighted software also includes the use of patented inventions, Facebook adds a patent license to assert that there are no submarine patents (that Facebook is not using copyleft as a means to setup future patent trolling). Everyone agrees this is a good thing.
3) The patent license is worded in such a way that it sets up a sort of 'patentleft'. Facebook is asserting not only that it's patents are defensive, but that there's a license which guarantees it. This license works kind of like copyleft does, though maybe with less precision (at the organizational level). It asserts that if your organization is using patentleft inventions, then by definition, all inventions that come from your organization are derivative works. Unlike Apple's open source license, the guarantees provided by patentleft have no bearing on your rights under copyleft. Not everyone agrees this is a good thing.
For patentleft, some people work in organizations that wish to split their efforts into those benefit from patentleft, from those that don't, like they currently can with copyleft. In the case of copyleft, it's a lot easier to detect infringement (does this code use copyrighted code). At the patent level, not only is it more difficult to tell, but the litigation around it is more difficult to settle.
I personally think that patents and copyrights, when done right, add value to society, but that current laws aren't "done right". Further, I think that copyleft and patentleft exist because of flaws in the existing system of patents and copyrights, are made to fix them, and I fully support that.
* Disclaimer * I am currently a Facebook employee, though my views don't necessarily represent those of the company. I wasn't an employee when the license was created, and I have no work involvement either in the license, or in software covered under the license. When the second/third clauses were added, I was a Google employee worried about the effects of these clauses.