Their leftism is mostly a hipster pose, a gesture of too-cool-for-my-pants nihilistic South Parkian smugness.
You never bother talking about how much, say, Huey Lewis and the News sucks -- they're too far away -- you're only interested in tearing down the obvious suckiness of bands that are RIGHT THERE.
Maybe they don't KNOW this about themselves, but it's still there operating at some level, & will come out if Bernie starts getting close to the nomination.