Well, the Constitution doesn't include any such limitation. You can try to say that it's "implied", but then you're using the same argument liberals use when they say the 2nd Amendment doesn't protect an individual right.
Excellent point on liberal free speech hypocrisy regarding abortion protestors, the same also applies to campaign finance "reform".
Could such laws be enacted locally (state/city)? I heard recently that Michigan enforced a profanity law when someone left a message with some colorful language on an answering machine. You could say that the caller was just trying to be persuasive in communicating hate . : )
Slightly off topic, and I've posted this a few times, but why is it that when we interpret the Constitution with external documents to see what the founders intended, the liberals like to dismiss the Declaration of Independence (because it's not the Constitution), but for some reason Jefferson's letter to the Danbury Baptists is so weighty. In fact, those words "wall of separation" have somehow been infused in the 1st Amendment. I'd bet if we did a Sean Hannity "Man On The Street" poll and asked a bunch of liberals where in the Constitution the words "Separation of Church and State" were written, 95% would gleefully respond, "The First Amendment!"