I grew up during the "latchkey kid" era, when it was pretty normal for fairly young kids to have to do superhuman tasks like navigate their way from the bus stop to home and then spend a couple of hours alone before a parent would show up. At the time this wasn't necessarily seen as a good thing, but concerns were much more about kids being lonely and lacking human interaction than they were about lack of adult supervision and safety issues.
Obviously things have changed quite a bit, and I'm often shocked at how old kids have to be before parents are comfortable leaving them alone. It's their call, but definitely times have changed quite a bit.
And, yes, it's perfectly safe to put your 9-year-old on the subway. It isn't a choice every parent would make, and that's fine, but it's a perfectly sound choice.
And the woman who did it gets it right, that accidents and bad things can always happen and since parents get blamed for them they feel the need to go through elaborate steps to shield children from very low probability events. I'm reminded of my college days, when rape awareness education for women was all the rage. It started off in a sensible place, but it also gave women a list of "risky behaviors" which made them feel responsible for their own rape if they actually did crazy things like walk out alone at night.
