Well, we'd be here all day with chicken or the egg logic but I'll indulge, they wouldn't have needed the canons if they didn't violate the law.
You really want to open that can of worms? That gif was from long before the pandemic btw. Afaik protesting is not against the law. Plus what do you do if you want to protest against the laws...
“U.S. police across the country are failing their obligations under international law to respect and facilitate the right to peaceful protest, exacerbating a tense situation and endangering the lives of protesters."
Who is violating the law...