You are drastically oversimplifying the situation here. The goal of a protest is not to be seen, or to be harassed by police. The goal is to effect change, and surveillance is a tool of the powerful that the United States has long had a healthy skepticism for.
I'm not going to go and complain about it because I'm not living in Europe, and it's not really my place to tell you that you're being over-surveilled. But if you're pointing at London and saying, "don't worry, we already have this" -- bad example, that doesn't make me feel better. I don't want the US to have the surveillance omnipresence that London has, that would be a negative outcome for me. I don't think London's situation should be treated like it's normal. To be blunt, I think your city has a over-surveillance problem, albeit not one that it's my responsibility to solve or draw attention to.
GP's analogy is gross and hyperbolic, it's not the one I would use. But there are some concerns I have here. A better analogy would be something like Baltimore's (thankfully ending) drone program, which creeps me the heck out even as I acknowledge that it's not obviously an invasion of a private space. The point of a protest is often to be seen, but not necessarily to be surveilled in that level of detail, and there are concerns that people should have about police departments paying special attention and putting special effort into monitoring those protests.