A lot of Americans are morbidly afraid of their children being away from adult supervision for more than a minute. I think suggesting 10 year olds ride a bike instead of taking a car or bus might infuriate more than a few people.
The issue was that she and 4 years old walked 20minutes to preschool and back. That is it, 40 min total walking time was assumed to be beyond 4 years old capabilities.