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If all they do is protest, carry signs and shout slogans, whose rights are they violating? After all, the Constitution (Remember that handy little document? We discuss it often here) provides a right to peaceful assembly. Until they unpeaceably assemble, protesters -- even if others consider them obnoxious -- are within their rights. The mere belief they may be disruptive is not a crime.
Just imagine how many people we could imprison if we start arresting people who we assume may someday commit a crime. And who would be the lead investigator, Nostradamus?