You would have to be naive to think that every supporter base is as bad as each other.
Go back over the last 10 years and the only stadium riots involved Canterbury supporters. The thing you are forgetting is these clubs represent a particular demographic, not just a jumper and conglomerate of football players. Take Penrith for example; the majority of their supporters are going to be no nonsense "westies" with blue collar jobs, modest bank accounts and a rental property. The bulk of the Roosters fan base are the latte sipping, beach going, poodle walking snob variety.
The fundamental difference in socioeconomic status of each clubs primary demographic, lends to unique behavioral patterns applicable to each area. The same way crime rates vary in certain suburbs, the general behavior of fans differs between clubs.