I'm not sure I agree that this is analogous to copyleft. Copyleft says that you must license all distributed dirivitives under the same license, but it doesn't

I'm not sure I agree that this is analogous to copyleft. Copyleft says that you must license all distributed dirivitives under the same license, but it doesn't affect any other code you produce, you could still sue Facebook for infringing on your copyright in unrelated works. Whereas Facebook's new custom license only grants you permission to use and distribute React if you never sue Facebook for patent infringement, even if the patent is unrelated to React.