How much of that is due to parents being paranoid about child molesters and freaks out in the neighborhood? I know when I was growing (in Korea) I did things that I would **NEVER EVER** let my kids do. Not because there were less child molesters back then, it just wasn't published all over the Internet/Cable News/Facebook/Google+ etc.etc.etc.
Just last week in Brooklyn, a kid was killed while walking 8 blocks from day camp. He was hacked into pieces and disposed all over the place. Friday was the last day the parents walked with the kid, and the following Monday was his first solo walk home. After reading that, you *have* to pause as a parent.
So it's not just about lazy kids. It's about lazy parents too. I supervise my kids while they play outside (I'm like secret service, scanning all over the place for danger). But they do play outside. Not all parents are willing to do the very basic... So it's a bigger problem than just "fat kids"
And don't even get me started on the GINORMOUS portions they serve in here in the states. It's grotesque.