Such a tragic accident. I think anytime something terrible like this happens, it is human nature to start looking for a concrete "reason", and that ultimately leads to blame. It makes us feel better, because as long as we can pinpoint a preventable cause, we can feel certain "it would never happen to me".
But the reality is that life is simply full of risks. We do what we can to minimize these risks every day, but every once in a while, you just get unlucky. It wouldn't take more than 30 seconds for a child to slip away and get into trouble. And how many of us parents can honestly say that none of our children were ever out of our sight for even 30 seconds?? Fortunately, the deck is generally stacked in our favor because of a multitude of safety precautions that society has put in place, but it is never full-proof, and tragedies like this accident will, unfortunately, never be non-existent.
Not to say that there aren't real cases of neglect and negligence (and to those I say punish them mightily), but too often I think the finger is pointed when there was nothing unusual going on with the parents. Nothing that the rest of us don't do as well. But we don't want to feel like it is even possible for us to end up in that situation, so clearly those parents must be different, and worse somehow.
All we can do is cherish the time we have, try to make the best possible decisions for our kids, and hope that we never end up on the wrong side of probability.