It's a difficult situation, because different people have different opinions on what the role of law enforcement should be in proactively addressing upcoming behavior that they believe may develop to be criminal. By engaging in targeted mass surveillance in ahead of a suspected protest, the police in this situation is literally preparing for a confrontation, contributing to an antagonistic relationship between (this slice of) the community and the police.
I don't believe this approach is productive, but I recognize some people hold a different opinion. However, when this approach causes knock-on chilling effects and reduces the likelihood that people needing help will call upon police [1], I believe it's a detriment to the health of society.
[1] http://www.theatlantic.com/politics/archive/2016/09/police-v...