Actually, from a legal layer it also might be a tactic to weaken the bargining power of anyone who uses React (why would Facebook want to discuss a patent portfolio with a company that cannot sue them for any form of patent infringement without losing their React protection). They also would lose their effective ability to counter-sue in a patent case.

I don't like software patents on an ideological level, but even from a legal and practical perspective these license terms should be reason for concern. The first thing I read when I look at a free software project is the license, and when I first saw React a few years ago I stopped looking once I read the PATENTS file.