i wonder: if you don't have a contractual relationship preventing it, can't fb revoke your permission to use their patents at-will? if so, has removing the patent grant actually changed anything?
It's actually the other way around, as I understand it. This is one area in which patent law and copyright law differ noticeably.
Due to the way implied patent licenses work, if Facebook distributes React without explicit restrictions or conditions, they're automatically granting a license to any patents that are inherently infringed by React itself. This is a pretty well-established area of US patent law, see e.g. https://www.wilmerhale.com/pages/publicationsandnewsdetail.a...
