I'm sure there's a rational explanation for this, but I have not been able to come up with one. In the United States, we drive on the right side of the road. That holds for both cars and bicycles. Pedestrians are supposed to walk on the left side, facing traffic. That lets pedestrians see oncoming cars and bicycles, and take evasive action if necessary.
On multi-use trails, though, pedestrians end up walking on the right side with the flow of prevailing traffic. This results in peds being overtaken from behind by faster-moving bicyclists. Thus ensues the endless debate of how cyclists should alert pedestrians to their approach, etc. Why don't pedestrians walk on the left side of the trail, the same as they would on the road? Cyclists and pedestrians would be facing one another, with each moving slightly to the left as they pass. Cyclists wouldn't have to continually call out "On your left!" Pedestrians wouldn't be startled by either the sudden appearance of a bicycle, or the disembodied (and ambiguous) command "On your left!"
Works for me.