Bike gangs

There was an article in the dutch newspaper about bike gangs, stating that they almost always have to do with criminal activities. It's the conclusion of some report of the dutch police. Why would this be? My husband drives a motorbike, but he is not like that at all. In fact, he used to work for the police. Is it the fact that it's about gangs and is the motorcycle thing just a coincidence? Or is it fed by old-school movies and is that report maybe only anout one particular gang? I wouldn't know. What I do know is that this stereotype is very often used in movies, so script writers take advantage of this image that lives with people.