Yes, if you like both loud music and breeze you are screwed. Though most people seem to do it for reasons listed above; next question, what is it with the whole cap backwards thing?
Well I personally like having the window open in Johannesburg. Its sunny 9 months of the year and the air is quite cooling and the music is always on. And Air-con is not as fun or tanning :). Its a summer thing!!
I've never questioned why people do it; I just assumed it was because they're amplification system was unable to make their music sound right at low levels (see the "Loudness Wars" for details).
Personally, I like hearing this kind of thing. I live in a place where an extremely harsh winter lasts for about 7 months out of the year, spring lasts for 3, and fall lasts for 3, leaving roughly 2 months for actual summer. Summer being my favorite time of year has become associated with seeing, hearing, and smelling certain things outside and being delighted that I can do so. The sound of music blasting from passing vehicles has become one of those things. Unfortunately, the smell of McDonald's makes me ill any time of year, as does the sight of morbidly obese people in small clothing. Well, we take the good with bad I suppose.