I don't get it either... I say on your left when skiing, biking, running ect. What gets me is when they shoot you a look or say something like you are an out of control freak. I have started passing people as if I'm in a car. Slow down to their speed, pick a safe opportunity, announce "ON YOUR LEFT" and get around them quickly and safely as possible. It stinks! What else can you do?

Be safe.