I'll call that a false equivalency, because only in my wildest dreams can I consider anything I'm working on equivalent to Microsoft. Among the Microsoft-level things I lack I'll count their legal teams, patent portfolio, and negotiating clout.
Anything I as a solo or mostly-solo dev would put together to sell would almost certainly be sold to a company less than 0.1% Microsoft's size - even at the 1% level it's just as likely that a company would decide they'd rather just build their own version or get someone to do so as a work for hire.
Edit: I'm also mentally picturing scenarios where Facebook's terms might end up tested in court. A developer team using an open-source framework without understanding the legal ramifications is one thing, and a thing done by people not authorized to negotiate binding contracts for the company. A purchase of a company or significant software (not off-the-shelf) on the other hand is by definition done by someone with authority to negotiate binding contracts. It's not that hard to imagine a scenario where that difference becomes relevant in court. Heck, I wouldn't be surprised if it turned out that Facebook had an intern or two somewhere focused on identifying and tracking projects using React and what companies they're owned by, just in case they end up in a conflict with someone and need to say "Hey, are they using anything of ours that can make this go away?"
Well, my point was that Microsoft has a very broad patent portfolio, as does Apple, and many other such companies that are using React in production. If they do not fear this patent clause, then I fail to see why your startup would. Especially if your startup has no patents at all (as is the case with most software startups). Apple & Microsoft would have a lot more to lose if they were somehow granting Facebook unlimited access to their patent portfolio just by their usage of React. You think their attorneys would let that fly?
I don't think their attorneys would let that fly, but I also think their attorneys most likely have already negotiated other contracts with Facebook that override whatever's in the React license.
This is not a tool for Facebook to be able to use against another multibillion major tech company. This is a tool for Facebook to be able to use against potentially disruptive tech upstarts of the sort that they were 10-15 years ago. Also, don't just think about Facebook's core business - remember that they're getting into all sorts of fields now. Do you think there are any VR related patents held by smallish companies that might be of interest to the owner of Oculus? If those companies are using React, Facebook now gets to negotiate with them from a position of "You've already basically given us permission to use your patented IP in our new headsets."
