Students at Brown threw a pie at Thomas Friedman, the over-rated NY Times columnist, during a speaking engagement, an act that Matthew Yglesias dubs "funny" and that Yale student Dara Lind defends thusly:
But there is no consensus that "the only acceptable public action is speech" -- there is clapping, booing, turning one's back to a speaker, holding aloft a Zippo (or, sadly, a cell phone) to signal approval, shaking your fist, giving a standing ovation... all of which, by the way, are more acceptable than certain kinds of speech (e.g. racist harangues, shouting at the top of one's lungs, etc.).
Throwing pies rankles, among other reasons, because it is destructive of public discourse. It contains less content than spoken dissent and imposes a far greater cost: prominent speakers aren't going to address college audiences if there is a likelihood that they'll get a pie in the face.
"Action - and especially performance — is a legitimate contribution to public discourse," Ms. Lind writes, as if any sort of action or performance is justified merely due to its physical medium! Should I ever find myself debating her in person I suppose she won't mind if I perform an experimental ballet I've developed called "Steal her microphone so the audience can only hear me," which ought to succeed even if I'm hit by a pie during the raid on her podium.