It's common sense rooted in the same concept of not entering others' houses and displaying bad manners.  You don't startle a bear by suddenly and without warning appearing in close proximity while its eating.  Bears don't like to share.  This caused a serious mauling two blocks south of my house in 2008--the victims were a female jogger wearing EAR-BUDS, and subsequently the bear, which was shot for defending its hard-won meal.  Nor is it sensible to jog wearing ear-buds in winter, alone.  This happened the following year to a young woman in Chignic who did so in view of three wolves.  They paced her a bit, then jumped, killed and partially ate her.  Two were shot.  The third was never found.  I'd bet a paycheck a significant number of these bicycling morons wear earbuds while hurtling down trails, so these young women did EVERYTHING wrong the single-track bikers do, with they exception they weren't on bikes.  And the bikers arguing to defend their stupidity REFUSE to connect the dots.