On the one hand "If" such curses existed, then the people of Carlisle are the offspring of those cursed which would account for the Downside of events for them, However the reality is there are no such things as curses, basically people have bad luck and they will attempt to find something to blame.
If it's not the stone to blame, then it would be the governing body, the mayor, the local constabulary, the dustbin men, traffic wardens, Tony Blair, heck the list could go on.
Heres another point, how many times have people "cursed" under their breath at people and nothing has happened? If "curses" were real, then we would all be living in a self perpetuating nightmare, one curse after the other.