Unfortunately that is true of some fans of these clubs, (not all though) I think once a club's fans has a reputation of hooliganism then it attracts even more of the hooligan element to watch them. Not footie fans at all, just thugs.
Some get labelled as trouble makers when they really aren't. The last time I was at Elland Road there was a surge of fans as we left the ground. I and a teenage lad were pushed into some policemen. We both said sorry and it was clear we had been pushed ourselves but the young lad was caught on film and cautioned. I and a few others said that it wasn't his fault, but the police didn't listen. I wonder if he stayed away from matches after that or the incident encouraged him to live up to his hooligan reputation...? Whatever happened it can't have encouraged him to look at the police force in a positive light.