We're not allowing governments to do the proper, necessary surveillance to control and prevent crimes in general, including ones at demonstrations/protests. Whenever these ideas start popping up, someone always finds some "privacy" concern that spoils a lot of tech-answers to crime and violence, including here on HN.
I find it very saddening as the victims of these crimes end up being victims that are ignored by "privacy" advocates and proponents, as if "privacy" is a bigger concern than violent crimes.