It's similar to the fall of 2011, when Bloomberg finally grew some balls and had the Occupy Wall Street camp torn down (The DUmmies' tears were particularly yummy that day); While he supported them at first, they quickly escalated to levels of violence and public disruption that made his city, and by extension himself, look bad, so he knew he couldn't turn a blind eye to it any longer.
Antifa shouldn't just be declared a gang: They, and fellow left- wing extremist group BAMN (By Any Means Necessary) should be officially designated as terrorist groups.