Not as straight forward as that though is it? , and even if illegal still needs to be dealt with sympathetically especially as the person thought to have committed the murder was a policeman, also other cities handled similar incidents sensitively. As football fans I’m sure we’ve all seen incidents which could have been handled better by our friends in blue.
It's called policing with by consent of the people and I suspect from what I have seen that the gathered crowd were far from consenting and a sad fact of modern life is that I would also make an educated guess that some very sad people actually went there to cause trouble and couldn't give a toss about the real reason that the majority were gathered there for.