The road sign mentioned in the letter does state that cars, bicyclists and walkers must share the road. This situation is true if there is no sidewalk in that area.
However, Arizona law states that if a sidewalk is present, pedestrians are required to use it. If there is no sidewalk, or the sidewalk ends, pedestrians may walk on the left side of the road or its shoulder, facing traffic
I don’t think that anyone has a problem with pedestrians on the road if there is no sidewalk, but if there is one, it should be used. After all, sidewalks are in place so that people don't have to walk on the street.