https://www.politifact.com/truth-o-meter/statements/2019/oct...
TV networks are required to air even untruthful ads as not to run afoul of regulations governing political ads. It's also likely that campaign laws requiring all political ads to be true would be unconstitutional.
Either way, the very fact that the Elizabeth Warren campaign is promoting a fairly blatant falsehood (and many people are happy to parrot a false talking point in the name of truth) is a symptom of the same problem that would force any reasonable entity to make the same decision Facebook just did. Truth isn't very popular, especially in politics and people (on every side of most political issues) would lose their mind if falsehoods were strictly banned.