Disclaimer: I don't have a settled opinion either for or against this surveillance. I really don't think I've heard all of the relevant facts or perspectives yet.

The main "pro-surveillance" argument that comes to mind is that sometimes these protests devolve into illegal looting and destruction of private/public property. I think my ideal would be for police to have only the surveillance needed to prosecute those crimes, and nothing else.

The "for" argument is outlined by Dataminr in the article. If someone with one of those old-timey "press" hats went down to the protest and started publishing information about where it is, how it's going, how many people there are, most people would consider that to be simple and non-problematic journalism. Dataminr doesn't think their behavior is any different.