Policing football is tricky.
You can have low risk Cat A matches that go off inside and outside the ground but Cat C matches go smoothly... like Millwall for example, all their matches were Cat C for a long time and heavily policed for good reason but as soon as it calmed down abit and the police decided a match was low risk (can't remember which) they dropped it to a Cat B, left them all to it and all hell broke loose... can't win really.
Unfortunatley London is where the money is... Derbyshire Constabulary like the rest of the county forces haven't got a pot to piss in and can't pay overtime to the officers needed to make it safe for fans so an agreement is made with the club.
London don't have that problem, loads of officers, loads of money, the terrorism threat (different pot of money all together)
Not only that but there's a possibility DCFC are paying the Police to police the town and areas around the ground to make it safer for fans when other clubs don't.
Times have moved on and it would be good if everyone could behave but there will always be bother at the football.
Would be a lot better in Franks pocket.