@ Jack Davies
I think the difference is that social landlords are regulated and monitored on performance and more than that, they tend to be responsible landlords and don't pass the buck when their tenants break the conditions of their tenancy. Nor do they wait for things to get so bad that the police are required.
when police are required they are called and most landlords work jointly with police to tackle ASB and youth nuisance.
In terms of cost, how many additional police would it take and at what cost to manage all instances of ASB if landlords decided that it wasn't their business. I'd argue the current solution is preferable as I'd rather we let police get on with more pressing matters.