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 collected here: name, address, duration of involvement. They'd end up in court immediately, and it would likely be found that their actions had a chilling effect which created a prior restraint on the public's constitutionally protected speech.
Yet online this kind of data collection amasses more data and is more chilling due secretive nature since you never know when you're not being tracked.