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It is a tragedy when our own negligence leads to something more horrible then we could ever imagine. But we must also take responsibility when it is our own negligence.
I think the question Weingarten was asking was, how much punishment is enough? What was so excruciating about the article was how much these people were hurting, and continued to hurt. They did take responsibility. I can't figure out how to say this without it sounding a cliche, but they'll live with this for the rest of their lives. I guess it's easier for me to imagine forgetting a child in a back seat for eight hours than imagining a parent or other adult relative (grandparents have done this too) who was simply able to shrug it off as a mistake. The people in this article weren't the sort who were routinely negligent with their children or even seemed to entertain the possibility of conscious negligence.
Sure, you could put them in jail for involuntary manslaughter, but what good would that do, other than to satisfy some abstract notion of vengeance and justice? It's not as if they deliberately left their children to die of heat exhaustion, so it wouldn't serve as a deterrent. And you would simply be punishing the spouse and the other children in the family by taking that parent away for a long period of time. It would likely cause more damage than it would prevent. They're not getting away with murder.
I suppose you could argue that there are people like the woman in the article I linked in my earlier comment who are cruel and have no conscience. But that sort of information would likely come out at a hearing or trial.