I definitely agree with your sentiment, but then I go talk to parents. Parents are protective of their kids. Some are loose, but others are not. I don't have kids, but have worked with them extensively, so I will refer you to some discussions around the issue:

http://www.blogher.com/9-year-old-rides-subway-alone-mom-her... http://www.nysun.com/news/why-i-let-my-9-year-old-ride-subwa... (also check out the comments on the original source http://freerangekids.wordpress.com/2008/04/06/why-i-let-my-9... )