If it is just a matter of the noose how is that not protected speech (however offensive) in exactly the same way that flag burning is protected speech? I'm sure there are many who find flag burning just as deeply offensive. Just as I would defend anyone's right to burn THEIR flag. I would depend anyone's right to display THEIR noosed rope. Is that how it works - just how deeply offensive YOU OR I find their display that determines whether or not it is protected speech?
There are two differences.
First, in order to be a crime, harassing speech must be directed at the listener. One must intend that the listener hear the statement. In the case of flag burning, it would generally not be directed at anybody in particular. If I go to a rally and burn a flag, I generally don't do so with the intention of harassing you as you sit at home watching the rally on TV.
In this case, the authorities believe (and will have to prove) that the display of the nooses was directed at the individuals waiting for the bus. Just driving around with a noose probably isn't illegal. Driving back and forth so that these particular people saw the noose probably is.
There may be a case where flag burning is not protected speech. Imagine I know that you are deathly afraid of fire and that you are terrified of being set on fire. I come to your house, knock on your door and, when you open it, I set a flag on fire. Chances are I'm doing it with the intent that you view the act. This would probably make it a crime.
The second reason is that harassing speech must have no legitimate purpose. A person burning a flag at a rally probably does have a legitimate purpose for doing so. He may want to communicate the idea that the concept of freedom is more important even than the symbols of that freedom. That's an important social and political point (even if you disagree with it). Since the flag burning has the purpose of making a political statement, it is protected speech.
In the case of displaying a noose, there probably are many times where it is protected speech. "Black people should up and leave" is a political point of view, albeit a stupid one. However, in this case the authorities believe (and will have to prove) that the display of the nooses was meant for no purpose other than to alarm the people waiting for the bus. If so, it is not protected speech.
The same is true with me burning to flag on your porch. In that case, I don't really care about political freedoms, I only care about scaring you. That is a crime.