Companies change. People change. You don't want it written into the license like that and depend on good will. If good will were enough we wouldn't have laws or government.
I’m not trying to be sarcastic, I just really don’t understand how these fears are justified. Besides, if Facebook decides they want to crush you and use React as a shield, you’ll probably have bigger problems in that moment than your choice of front end libraries. A bigger problem, I would think, is that you have something worth Facebook thinks is worth stealing. Facebook doesn’t need a React patent clause to smother a company.
Imagine a tiny company finally figures out UX and solves some of the problems with spam, load balancing, etc. on a decentralized social network. If the cat gets out of the bag, they're an existential threat to Facebook.