It's 1A but it's also SCOTUS precedent. If NYT v. Sullivan were to be revisited, these limits could change. Justice Thomas has expressed (on multiple occasions, though I don't have the opinions handy) that he wants to do that. Personally I think it should be revisited. The recent case of a Duke volleyball student claiming that racial epithets were yelled at her during a came turned out to be false, but not after the usual cycle of social media enabled outrage. I bring it up because it's another instance where all of the facts are easily verifiable, and journalists seem to not care to actually look at them and report what actually happened.

Well we can't have SCOTUS following the suggestions of racists like Thomas. /s?