you don't have to yell out "on your left" as long as you give them a good amount of room. make sure you pay attention to how they ride before passing them to avoid them randomly swerving directly into you. calling "on your left" usually confuses a lot of people and all they hear is "left".

if there's no one in the oncoming lane and someone is trying to pass me i don't see a reason to move or slow down.