It's because it covers any unrelated patents, not just the ones related to React. You can't sue Facebook or its subsidiaries, like Oculus or Instagram, for pate

It's because it covers any unrelated patents, not just the ones related to React. You can't sue Facebook or its subsidiaries, like Oculus or Instagram, for patent infringement, e.g. if Oculus were to infringe on your VR or AR patent, without losing a grant to any patents that may cover React owned by Facebook. It's unknown that Facebook has any patents for React, but if it did, it could countersue you simply because you rightly assert your patent against their infringing technology. If you own a company or are part of a university that has a patent and Facebook infringes, then you can't really use any of their open source products that have this specific grant, rather than not being able to use the specific product that they are infringing upon, if any. It's their prerogative to disallow that, but it goes against the spirit of open source, in my opinion. I speculate that this grant was probably written in this way to ward off NPEs, but I doubt any of those NPEs actually would use React and so in reality it causes issues for pretty much anyone else that cares.