If Facebook had not included this Patent grant and had released React under only the BSD license then any user would be in the exact situation which everyone is complaining so loudly about being in IF they decide to bring a patent action against Facebook. Specifically, you would be open to being sued by Facebook for violating a patent which they own.
What this grant says is for one specific circumstance (you haven't brought a patent suit against them) and for one specific limited set of patents (those related to React), Facebook will not sue you. If you like that protection then don't sue them. If you decide that you do want to bring a patent suit against them then you're right back where you were to begin with. Your one small limited protection is removed and Facebook can once again sue you if you violate one of their patents - just like they could before you started using React in the first place.
This business about it being asymmetrical is IMO a distraction. What would it mean for it to be symmetrical? That it would only trigger if you sue them for a patent violation related to React? What does that mean? You don't hold React patents, Facebook does. How would you sue them for a violation of a React patent? It makes no sense.