It's probably what he/she got use to doing when young. A lot of toddlers on their first bicycles will follow their parents as they walk the neighborhood streets. So they assume it's normal.

Just last week, I did see a mother walking with a child riding a bike. She was walking on the wrong side so the child on the bike could be on the correct side.

I walk quite a bit with my wife. And we walk against the motor vehicle traffic as ordinances tell us when on roads, However, I don't have a issue walking with traffic. And I'd be happy to see the recommendation changed so that everything in a lane travels the same direction. Then maybe the teens and young adults that get loose on a bicycle without knowing the rules will be pre-conditioned to travel in the correct lane.

Bicycles traveling against traffic do account for a significant part of cycling accidents. Almost a third of all bicycle accidents that involve another vehicle are cyclists going the wrong direction.