Slander and libel are handled in the civil courts. If I say, Beckhem can't play the game of soccer, Mr. Beckham can sue me. Fire in a crowded theater is immediately dangerous, accent on immediate. Advertising is commerce.
I'm not saying you're wrong. At all. But our First Amendment says Congress shall make no law
As for the penalty, from this side of the pond jailing and getting kicked out of school for being a drunk dick on Twitter, I see that as excessive.