This is something I really don't understand. Do they understand they're on a multi-use path, and that bicycles are very common, perhaps more common, than pedestrians? Once I approached a pack of peds and announced my presence. One of them panicked and yelled out "Oh my god!"

My favorite way to indicate passing is when riding at night. My commuter path is lined with those poles that have reflectors on them. As I approach a cyclist, I switch my headlight to blinking mode, and all the reflective poles ahead of me blink (as well as any traffic signs). Everyone has veered to the right when I've done this.