Slate - Why are protesters treated as threats?

A disturbing trend - protesters at presidential events are being singled out by law enforcement as "threats" against the president's safety and being forcibly removed, or even arrested:

On Friday, the antiwar-T-shirt-clad mother of a slain soldier was pulled out of a Laura Bush speech in New Jersey and threatened with arrest. A West Virginia couple was detained by the Secret Service for wearing anti-Bush T-shirts at a July 4 rally - they filed a lawsuit last week - and AIDS activists were removed and kept away from reporters at a Sept. 9 presidential event in Pennsylvania. Most notably, some 1,800 protesters, monitors, and passersby were jailed in indiscriminate raids during the Republican Convention in New York, while several hecklers were dragged off the Madison Square Garden convention floor. All were arrested or threatened with arrest, and hundreds expect to stand trial....

Because this is the 'age of suicide bombers,' the Secret Service should have latitude to get rid of anyone suspicious who is standing near the president's route, [Judge] Marchant said. Fair enough. But [this protester] could only have been singled out for suspicion by one thing - the slogan on his sign. Even allowing the strange notion that the Secret Service expects terrorists to begin their assassination plot by carrying a noticeable antiwar placard, it's enough to make anyone with a dissenting view think twice before deciding to stand out from a crowd.