Because I've seen quite a bit of it. The DKs are, or were, unique in that they actually talked about rich people and rich kids...however, the funny thing is that in my experience the people who have been most impressed with the DKs perspective, and have taken it most to heart haven't actually been people from working class backgrounds but those from middle class ones, who have become activists or some such. I think there's a very good reason for this, and it has to do with Jello Biafra's class origin itself.
Biafra didn't come from a working class background but was the kid of two librarians working for the University of Colorado at Boulder. Presumably, they at least had bachelor's degrees, and they exposed him to a lot of sophisticated culture more typical of the middle class than the working class. His perspective was shaped by seeing rich kids come to the University of Colarado while his parents as staff at the University didn't do as well. This is quite different than the experience of most working class people.
The United States is extremely class segregated, and although the situation might be different in smaller communities, in general working class people don't really see the rich and the powerful close up. They're there, out there, somewhere, but live in worlds miles away, or in the next county. They're represented possibly by parent's bosses, or their own, but even then these people may just be the employees of the owners, of the truly rich, who remain absent and not rarely seen.
Because of this, although there may be class resentment on the part of working class folks, it isn't shaped by as much detail as is reflected in the songs of the DKs. Biafra and company document the foibles minutely, and besides special cases like kids who grow up in university towns, the only other people who see these folks as up close and personal, and who cultivate a resentment towards them, aren't workers but members of the middle class itself.
Working folks aren't the only ones who resent the rich. They're also resented by middle class people who have solid middle class values, who don't really care about workers or the working class at all, but who instead resent the fact that there's this stratum of folks above them who have more privileges than they do. It seems that many of the middle class fans of the Dead Kennedys are children of these people, who have seen the rich up close and personal, and resented them not because at any time in their lives they wanted for anything, but because they don't have as much money as them. This is quite different.
Surely, the parents of some of these folks are working quite hard to join the ranks of the very people whom they resent. The problem isn't money itself, potentially, just that they don't have it yet and they want it. The kids take a different approach, and see their class consciousness raised again, not by the fact that as children of business owners or lawyers they ever wanted for anything, but because in relation to the rich they had to do some sort of token job, or had to contribute money to their parents buying them a car instead of them paying for all of it.
This is not class resentment so much as greed resentment, resentment not from the perspective of oppression but from the perspective of upwardly mobile professionals who believe that those above them, who have things that they want, don't deserve them, and that they do.