There are two things here: The copyright license, and the patent grant. Copyright applies to the concrete implementation. You have to agree to the license to be subject to it, and to legally use the code.
A potential patent applies to any implementation. Even if you write a clean-room clone of React, if it uses the same patent, Facebook has a patent claim. But that means the patent grant is not specific to the code; it doesn't even require consent, Facebook could allow you conditional patent usage even without your knowledge! A corollary is that you are strictly better off with the patent grant, it imposes no additional constraints on you.
License with no patent grant: Facebook can sue you for infringing patents, even if you are using a clone!
License with patent grant: Facebook cannot sue you for infringing patents, unless you do it first.
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Second, I think the philosophy behind the patent grant is twofold: 1) that software patents are not legitimate. Enforcing a patent is not seen as a legitimate right, but an annoyance, like pissing on someones lawn. From that point of view, it seems not asked too much from somebody to refrain from doing that. (I don't know if that was the idea of the people who drafted that license, but it wouldn't surprise me.)
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Another, unrelated observation (and please don't invalidate the first observations if this one is wrong as internet commentators are wont to do):
I see nowhere in the license [1] that it requires you to take the patent grant. Is that true? It would be silly to refuse it, because you are strictly better off with it, of course.