As kasey_junk implied, this sort of over-protectiveness is not universal in the USA. For example, NYC provides all students that live further than 1/2 mile from their school with a discounted public transit card (soon to be free). While very young kids are rare, tween and teen students can often be seen unaccompanied on mass transit. And even more often walking alone or in groups without parents. No one bats an eye except possibly to complain a bit about the groups of boisterous kids blocking the sidewalk :-)