Why do people walk on a bike path when a pedestrian lane is clearly labeled next to it?

All right, I know this sounds petty, but hear me out. Several of the places I travel regularly have separate bike and pedestrian paths right next to each other, divided by bushes or curbs. They are extremely clearly marked, yet I continue to see pedestrians regularly walking in the bike path (e.g., this guy on the Santa Monica beach path).

What would ever possess someone to do this? They gain no advantage that I can conceive of by walking in the wrong lane, and it’s inconvenient and/or dangerous to the bikers to have them there. A few days ago, I was riding my bike to school and two girls were walking in the bike lane. When I was about 5 feet away from passing them, one of them, who was talking on her cell phone, randomly took a gigantic step to the left, forcing me to brake hard, swerve into the bushes, and eat pavement.

Now, I concede that I have on occasion veered into bike lanes temporarily while running if the crowds in the pedestrian lanes are impassable. I think this is justified if a runner’s pace is closer to the average cycling velocity than to the average pedestrian velocity.

So the situation that continues to baffle me is: why are the pedestrians in the bike lane if the pedestrian lane is not (a) insanely crowded and (b) moving at a substantially slower pace? Can I stop and ask them sometime? Is it rude if, as I ride past, I gently suggest that they move to the pedestrian lane?