Why can't kids play outside on their own these days?

I was just reading an article in the paper with yet another comment that went something like this: "Of course, nowadays, you can't just let your kids play outdoors on their own". No explanation of why this is, no commentary that it's a bad thing, nothing. I think it was something to do wtih why kids are getting so heavy nowadays.

Here's my take on it. When I was a kid, we spent whole days outside in the neighborhood, in the woods behind the houses, at other kids houses, etc. It wasn't a problem, and it was fun. Nowadays, what with people being paranoid about one thing or another (child abduction mainly?), letting your kids wander about this way, without knowing exactly where they are and what they're doing, is bad parenting. I think that people are just getting more and more paranoid, and that unreasonable standards of safety are being set.

Why is this? Is it going to change? My son Kenny just turned 2 years old a month ago, and I want him to be able to go to his friends house on his own in a few years, without me being thought of as a bad mother.