From some of the comments above it sounds more like every Japanese adult is a guardian.
Assuming they actually do love their children, the reason the parents do this seems obvious enough. They're stupid.
Says the man who lives in a country where his own kids aren’t safe playing outside the house.
In the country where I was raised the rule was go out and play, just be home when the streetlights come on.
If you got into mischief, ‘a little birdie’ would tell your mom, and there’d be hell to pay when dad got home.
Are you happy living in a country where you don’t know your ‘little birdie’ neighbors, where no one cares if your children are damaging someone else’s stuff, and there is no working dad coming home in the evening?
You are? Then by all means, do criticize the Japanese...