The way I read it, it's not evil.
It's a known and deliberate shortcoming of many licenses (e.g. BSD) not to include patent stuff because it makes everything unnecessarily complex. There was recently an article about why BSD and MIT are so popular, and it's because they're concise and understandable. There is a reason WTFPL exists and some developers resort to it as a way to avoid legalese.
Facebook clearly was aware of this "shortcoming" and being a big player, they might have wanted to be nice and say "we won't sue you for patent infringement if it turns out we have a patent on something React does". Then the managers went "but what if they sue us? Patents are not only for offense but also our defense, we would weaken our defense." And so the clause of "except if you sue us first" came into being.
And now this fuss about the patent part making it not an open source license? Oh come on.
I really don't like Facebook as a company, but this bickering is silly.