Leeds was the first away ground i got a police camera shoved in my face while going in and out of the away end in the late 90s. Wasnt very nice tbh, treat like shit before youve even done anything.
And that's the problem, treat people like criminals and suddenly a minority of them will behave that way. If you're a young, impressionable man travelling the country with your mates, getting boozed up, and then the police are herding you places, filming you, watching you like you're about to do something they can arrest you for, you're infected by the idea.
I'm sure there are a small group of away fans in most clubs who love that feeling. For the majority though it's ridiculous, and they just want to go and watch the match with their family and friends. It's a lot better now, and it'll keep improving thanks to initiatives like FSF, but when it was really bad (before I ever went to matches) you wouldn't want to take your kids, would you.