I've had just about exactly the same experience on one of our local, well-used MUPs. This despite signs all over the place *reminding* people that it is a MUP and to keep right at all times, control your dog, etc. I don't understand what's so difficult to comprehend about "keep right; pass left only."

More often than not, too, when I've approached walkers or in-line skaters and announce (in as nice a non-startling tone as possible) "on your left" the people actually move to their left...right in front of me.

People can be funny (but not ha-ha funny ).