Just because it's never happened before doesn't mean that it couldn't. If you read accident reports so often, I'm surprised that you cannot see how this could happen. It's an accident, they didn't chase her down and run her over....
Inexcusable is driving drunk.
Inexcusable is leaving your kids in the car while you go drink and gamble.
Inexcusable is being cracked off your face and rolling on your kid in your sleep.
This was a tragedy, plain and simple. Could it have been avoided? Maybe, but at the expense of a fast response and saving other lives? Possibly...