That, according to the WaPo's Dana Milbanks:

I have to say the Secret Service is in dangerous territory here. In cooperation with the Palin campaign, they've started preventing reporters from leaving the press section to interview people in the crowd. This is a serious violation of their duty -- protecting the protectee -- and gets into assisting with the political aspirations of the candidate. It also often makes it impossible for reporters to get into the crowd to question the people who say vulgar things. So they prevent reporters from getting near the people doing the shouting, then claim it's unfounded because the reporters can't get close enough to identify the person.

(h/t Romenesko)