I assume that the thinking is the along the lines of "serious runners" love running so much that they don't need any other distractions or entertainment while running, and anyone who would prefer to listen to something rather than savor the purity of the run clearly isn't as serious.
Personally when I'm doing more serious running--like speed workout days--I don't want to be carrying anything extra or wearing anything extra. Also, when I'm doing short runs I value simplicity and don't want to complicate things by messing around with any extra gear or equipment. So usually the only time I listen to anything when I'm running is on long easy run days, because that's the only time that I'm running long enough to feel like it's worth messing with extra gear and when I'm not trying to run hard enough to care about wearing extra stuff. I guess the other time I would be likely to listen to music is if I'm in the gym on a bad weather day running on the treadmill or trying to take it a little easy on the arc trainer.
Slightly off topic, but what annoys me is all the people who listen to music while racing. Why even sign up for a race if you're just going to isolate yourself in your own head and not be open to interactions with the people around you? You might as well save yourself the money and hassle of dealing with traffic and parking and go run a time trial by yourself. I'm the most anti-social loner most people will ever meet probably, and even I recognize that racing is social experience and wouldn't want to ruin it by isolating myself from it.