I am a parent of 2 teenage boys (16 and 18) and this is similar to the philosophy under which they were raised. Contrary to what people are saying in the posts, it is not difficult to raise your children this way, nor more dangerous. The worst injuries either boy has sustained have all been at organized activities. The benefits, and I can't overemphasize this, outweigh whatever the hazards.
The problem isn't the kids of course, but parents, who are extremely overprotective of their children. I blame our media, which focuses on the violence and dangers in our society to make money while ignoring we're living in literally the safest time in history. They've scared the parents into thinking the world is too dangerous for their children.
It's the kids who suffer. They don't get the learn autonomy, or how to use their imagination, or how to set their own boundaries. And most of all, how to productively manage boredom. I see how they're doing that these days - they've got their noses in phones.