Slam dunk case, IMO.
Not exactly.
The Constitutional right to Free Speech does not include a duty to provide a venue. The real Constitutional issue is that the people who wanted to listen to Milo were denied their right to Peacefully Assemble and address their grievances. Even the protestors had a right to assemble and exercise their right to Free Speech. The operative word here is "Peacefully."
You want to look at the Mayor's duty to maintain law and order. And when he failed in that duty, look to the Governor and his responsibilities to maintain law and order when the locals fail or are overwhelmed. Unfortunately, the solution to that problem lies with the ballot box and fat chance of that happening in a uniparty state.
I loathe the tendency to Federalize everything But if there is a Civil Rights issue here, then it is the Right to Peacefully Assemble.