This kind of surveillance is not very effective for preventing or solving crimes. It is much more effective at making lists of people willing to lawfully and peacefully stand for the human rights or others, -- "potential subversives", as the parties monitoring might call them. Imagine, instead, if law enforcement were to stand outside of a marijuana-legalization rally and collect the sort of data effectively collecte…
I'm not sure the crime solving utility is wholly relevant here, police have other duties like maintaining public safety. The obvious utility seems to be monitoring different protest zones for signs that violence is beginning to break out. If x people are suddenly tweeting about a fire or gun shots near an active protest then you know it would probably be wise to at least send a scout over there to evaluate the situat…
Let's take a moment to determine how likely it is that the cops will stop sending small battalions of riot police into situations where they are highly unnecessary. As protests have gotten smaller in many cities, police have started sending riot police that outnumber the protestors. They have shown no interest in measured responses.
It would be nice if the police used data to reduce their use of force, but that appears to be antithetical to their way of thinking.