That's a strange thing for Facebook to do, for a couple of reasons.
First of all, after the 2016 elections, and all the scrutiny over things like Cambridge Analytica's misuse of data they skimmed from Facebook, you figure Facebook would be extra careful to at least give the appearance of wanting to be clean. Allowing ads based on false information will be another blow to their already questionable reputation.
Secondly, why do even more to piss off the democrats? Allowing Trump to purchase ads on facebook to promote his lies may give Trump some benefit, but there are strong indications that Trump may be headed to defeat in 2020, even with Facebook's help. The didn't like Warren before, how friendly do you think she'll be to facebook's interests if she wins the presidency despite Facebook hosting dozens of ads accusing her of all sorts of conspiracy theories?