There is apparently a subculture of people who decide to economize on the car part of the "loud stereo in car" formula (since they really don't care about the car—it's all about the music) and put their loud stereo on the back of a bicycle instead.
This quotation from the article caught my attention:
"People say, 'It's the next best thing to having a system in a car.' But it's better because you don't even have to roll down the windows."
I had been unsure what to think about people who drive down the street with their stereos blaring. Are they audiophiles who prefer their music loud? Or are they jerks who like to annoy other people with loud music? That quotation sort of settles it.