But they were already violent, just to the peaceful protesters. So why was there a massive police force at a peaceful protest, where they were already willing to brutally engage with the protesters, but not in soho where there were looters two days in a row?

Your argument can't possibly be "Because they didn't want to escalate" or "They didn't want people to think they're violent" when they did escalate and were violent elsewhere.

I am in full agreement that it was tactical - it allows them to punish a neighborhood and to shift the story.