Why are little kids in Japan so independent?

It’s a common sight on Japanese mass transit: children troop through train cars, singly or in small groups, looking for seats.

They wear knee socks, polished patent leather shoes, and plaid jumpers, with wide-brimmed hats fastened under the chin and train passes pinned to their backpacks. The kids are as young as six or seven, on their way to and from school, and there is nary a guardian in sight.