tl;dr: React’s license is asymmetrical [0] therefore FB. is being an evil corp. that wants to destroy the web [1] :)) (tinfoil hat on)
[0] - HN thread (first comments explains nicely the patent asymmetry) : https://news.ycombinator.com/item?id=14779881
[1] - GitHub issue still not closed: https://github.com/facebook/react/issues/10191