And good for them, but that doesn't affect the fact that they are doing you (/me, us) a favour, an act of supererogation.

In any case, my point isn't really about whether they are being kind or not. That's a fortiori. It's that they certainly aren't under any obligation to provide it, nor to maintain it on account of having provided it.

(Although to respond to your precise point: I'm severely doubtful of your claim that Facebook gets 'huge marketing PR' from an open source library. No one is signing up to Facebook because of the [apparently-existent] 'huge marketing' of the fact that they wrote React.)