The point is that if you're in direct competition, the chances of Facebook violating one of your patents is much higher, and then your cost of suing them is significantly increased (since you also have to rewrite your app).

If you're ok with trusting a direct competitor not to misbehave, then sure, use React under those terms while competing with FB. But large companies consider their patent portfolio to be a significant strategic asset, so it shouldn't be surprising that their legal counsel advises against accepting such a clause.