The walk left is a carry over from rural road thinking, whereby pedestrians are assumed to be safer facing traffic, whereby they may dive for cover if about to be hit. This thinking may be why many new cyclist feel safer riding against traffic.

While I agree that pedestrians walking facing traffic makes sense on roads, I'm not sure it has the same benefits on MUPs, though IME it often does seem easier to have those I approach facing me and seeing me at a greater distance.

For me as a cyclist, it barely matters which way a pedestrian to my right (his right if going the same way, his left if facing me), since at 3mph or so, his speed isn't a factor. OTOH- by walking left, pedestrian can see me coming and it spares us both the "on the left" and "bicycle behind you" nonsense. So it makes sense on paper, but the problem is that it's not well understood and so can create confusion.

I'm solidly agnostic on the question, probably because IME pedestrians will be walking right, left and center anyway.

If I could have any single rule followed it would be "keep your children and leashed dogs, between you and the path's nearest edge". But of course, there's no hope of that.