And it's a perfectly sound conclusion to reach if you're actually familiar with the protest scene. I call it a "scene" because that's exactly what it is. Just like you have ravers, goths, punks, emo kids, hippies, and whatnot, you have protesters. And it doesn't matter if he protest is to free Mumia, no blood for oil, food not bombs, or whatever Noam Chomsky's latest book is about; it's always the same people. That's because it's their social circle. The weekend's protest is where they go to hang out with their friends, to have fun, to see free concerts, to get dates, and so on. (Yes, when I first moved to San Francisco; I used to hang out with some of these people myself.)
So yes, when a bunch of strangers show up, nobody knows who they are, and they start causing trouble; it's pretty damn obvious... if you've paid a whit of attention at the protests every weekend previously... that what you're dealing with there is not protestors, but that the cause of the week has touched the nerve of someone powerful and they sent in the agents provocateur.