Bruce Schneier notes that Montreal police have been tagging people with invisible ink.
Okay, so you can tag people with ink. In the U.K. police have been known to make house-calls and just keep prominent protestors at home. In the U.S. police have been taking pictures of the crowds. The arrest records alone mean there are records, and there are very likely records of people who haven't been officially arrested.
So, I can reasonably assume the police and state agencies can defuse any protest within a week, with no headline grabbing confrontations. Just pick them up one by one coming out of the local Starbucks. Charge them with trespassing. They may even be able to hold them for a day or two. Round up the usual suspects as soon as the call to protest goes out on Facebook. We already know the media won't be listening- they are the ones pounding the protestors-as-scum message.
So why do they even let protests happen? There are probably more reasons than this, but I can think of two right now. They want people to think they are still free, and they also want to be made to look intelligent by comparison. I actually think the Republicans benefit most by the latter reason, though I suppose some Democrats could be using this in an attempt to waste more money. They both, however are using the first notion- they want us to think if we vote, protest, etc... if we stay inside the system and use system approved approaches to change, we will somehow change the system.
I'd rather be free and have no voice, than have voice and not be free.