How would that help? If Facebook has any patents on React, it's quite likely that preact is violating them. If you're using React then you're protected by the patent grant. If you're using preact then you aren't.
Well, please correct me if I'm wrong, but as I understand it by using React you're essentially giving up your right to sue FB for infringement of any of your patents (assuming you wish to keep using React). True, by using something like preact I suppose there is the alternative risk you pointed out. But perhaps it's a tradeoff some businesses might find acceptable.
You don't give up the right to sue them; you just lose the patent license if you do. If you use preact, you don't have the patent license at all, so you are in the same situation as if you used React and sued them over patents.