They sell an image? I think that stopped in the 80's. There are no more Jack Tatum late hits. They don't bring in trouble makers anymore (with the exception of Randy Moss). All of Al Davis' court battles have been settled. Look at the guys they drafted, for crying out loud. Every one of them might as well be choir boys!
And I think there is a difference between selling fans on a brand and stereotyping all fans as quintessential embodiments of such a brand. Even if the team played the same way they played in the 70's (the offense actually did play that way in Art Shell's second tenure. But I digress...)
Even if the team did play with the reckless abandon that they played with in the 70's, there's no need to take it two inches farther and include the fan base. Every fan base has got some rowdies in it, just like my Raiders do. But, because of the costumes and Oakland being a rough city, our fan's transgressions get magnified.
Every fanbase has rowdies in it, as you say. The difference is that 80 percent of the guys outside the Bay Area who are Raiders fans are there BECAUSE of the rowdy image. The Raiders made a crapton of money selling an entire generation of kids on the outlaw WWF thing -- "We're the team of badasses and late hits. Buy our jerseys." And it was very successful -- if you grew up in the south in the 70s, and you didn't live in an NFL city (as most of us didn't), then you were either a Cowboys, Steelers, or Raiders fan. And none of the guys who were Raiders fans picked them because they were particularly good. It was all because of the image they were selling.
And so what you have now is a national fanbase that includes a ton of these same guys who have grown up and turned into these fatass, middle-aged versions of Beavis and Butthead. And for them, the brand and the image is still as bright and shining as ever. Which is why, when the Raiders play in your stadium as a visiting team, it's like a convention of the ancient and balding and drunken Kiss Army has come to town. Committment To Excellence! Just Win Baby! Fsck Yeah!
You may be too young to have seen all of that at the time, but let me tell you straight up -- the reputation that hounds everybody with a Raiders shirt on today maybe unfortunate, but it's not unfair. The reputation of their fans is well earned. And it's something that Al Davis cultivated deliberately, and enriched himself greatly doing. This is why all the brand-manager types make a pantload of money: branding is enormously powerful, and, once established, it basically lasts forever. In ways both good and bad.
EDIT: I should note that one of my good friends is a 40ish guy who grew up in Knoxville and supported the Raiders. (And still does.) He owns his own software company, is intelligent and successful, and is in no way a fatass, middle-aged version of Beavis. He does not dress up like a pirate or a biker when he goes to see the Raiders play. However, as he will be the first to tell you, he DID pick the Raiders as his team when he was in elementary school entirely because the badass image. And just because he didn't go the principal's office > smoking area > Camaro > dressing-up-like-a-pirate route doesn't mean that a whole bunch of his fellow pre-teen 70s Raider-loving brethren didn't. Just go to any average Raiders away game on the east coast and look at the average age of the guys who are dressed up and acting like idiots.