Interestingly, most of the posters correctly identify the core concern. If you've invested significant resources into developing React projects, have IP, and Facebook infringes your IP, you're in very hot water.
Most businesses aren't competing with Facebook or patenting business critical innovations in spheres Facebook will be operating in. Using React in all of those cases is a non-issue.
For these people, the license's restrictions seem very minimal, so dumping React in the Open Source bucket seems reasonable.
For others, this restriction is an existential threat. This is why other people have an issue with the license being considered open source.