Seems that the real issue is that Facebook asserts/implies they have patents relevant to React, and that they are willing to enforce them in court under certain conditions. This creates an uncertain situation. The patent grant helps somewhat, but limits your freedom of action should Facebook e.g. steal your technology. Look at the four possible scenarios:

    Facebook | Your Company | Outcome
    
    Grants patents conditionally | Does not sue Facebook | Things hum along nicely
    Grants patents conditionally | Sues Facebook | You're liable for patent infringement
    Doesn't grant patents | Does not sue Facebook | You're liable for patent infringement
    Doesn't grant patents | Sues Facebook | You're liable for patent infringement

The patent grant is fine if you think you'll never, ever want to sue Facebook. But should a dispute arise, you'll quickly find the technological rug pulled out from under you. You've now built your entire business on Facebook's asserted IP and have no recourse but to license it or try (futilely) to defend your infringement in court.