Without an explicit patents grant, you're not granted any patents and therefore could be sued for patent infringement if the software is covered by any patents owned by Facebook.
Also, I think the "license" (in the legal sense) for React consists of both the LICENSE and the PATENTS file, so in order to re-distribute the project you'd still personally need to obey its terms.
There's also the problem that the requirements for a "derived work" (which could be distributed with your own license) are more complex than "change a bunch of files", so if you simply slap a new license on someone else's project the new license is not necessarily enforcible and you may be infringing their copyright.
Basically, IP law can not easily be gamed (case in point: legally speaking there's nothing wrong or unexpected about patent trolls).
Also, I think the "license" (in the legal sense) for React consists of both the LICENSE and the PATENTS file, so in order to re-distribute the project you'd still personally need to obey its terms.
There's also the problem that the requirements for a "derived work" (which could be distributed with your own license) are more complex than "change a bunch of files", so if you simply slap a new license on someone else's project the new license is not necessarily enforcible and you may be infringing their copyright.
Basically, IP law can not easily be gamed (case in point: legally speaking there's nothing wrong or unexpected about patent trolls).