See, the first amendment protects this right:
the right to peaceably assemble
Now, as I am not 200+ years old, I don't necessarily know what the men who wrote that meant, but as a logical man, I'm going to say a bunch of people blocking public road ways and acting in a way that causes or allows for chaos and destruction to reign is not a prime example of the first amendment.
Now, I am in no way condoning everything the police department of Ferguson is doing, however as human beings with self-preservation ruling above all else, I can see why they'd feel the need to disperse crowds that could be considered violent, disruptive, and potentially dangerous.