I believe it's because last weekend cost them (I think the figure was) £300,000 in policing costs, and that doesn't begin to look at the economic damage. And then there's the poor cop who got a flare to the face. The city can't afford that to have that go on. And, from what I'm reading (and there is absolutely no way to know how accurate that is) the tensions are ramping up badly. Supposedly there are police infiltrators on both sides gathering that info? So, let that go on and deal with the aftermath, maybe some cars/buildings burnt or people killed, or prohibit it? Philosophically, yes, everyone should have a right to peacefully march to express whatever cause they happen to espouse. That would mean that they'd also have to let Nazis or the KKK march if they applied for a permit. Distasteful, but their "right" under the law here. Realistically, however, this is not working. It's a dysfunctional situation and leading to serious public disorder. Peaceful demonstrations are one thing. Riots are not in the same league.
I'm not sure it's terribly easy to arrest every rioter, either. Do you arrest the guy who first throws the brick that hits the other guy in the head, or the people who then respond in kind? Given all variables, I have to agree with them that it's best to just prohibit the marches. If they were "just" marches, political or otherwise, I'd have no objection (although the business owners on the routes may have, and people trying to use the public transportation that gets discontinued). These are marching provocations, and violence is becoming more common and more serious. I don't care which side is marching - although it does appear that the violence is pretty much targeted at the Irish from the Orange orders. So do they just summarily put all the Orange order types in jail for the weekend, and then give them the statutory slap on the wrist that passes for consequences here because they can't play well with other children? At what cost?
In kindergarten it was always that things are taken away if the kiddies can't share/play well with them. I'm not sure that's not a good idea for this. Again, if it was just marching, that's one thing. Gangs of thugs attacking the marchers and the ensuing riots is quite another.