How do you figure? Lots of parents have been prosecuted for leaving kids (and here in California, pets) in cars.
It's reassuring to assume these tragedies are intentional because each of us is sure s/he will never do such a thing.
But the truth is probably more alarming: Americans work more hours per week and more weeks per year than any people in the developed world. Studies show Americans are chronically sleep-deprived, despite our huge consumption of sleep aids. Both parents usually work and, with a 50% divorce rate, we have a lot of single parents who share custody with the other parent.
Any time a task is intermittent, it's easy to lose track of when the task is to be performed and whether it has been done. If you have the kid on Tuesdays, Thursdays and every other weekend--and you are distracted by what happened at work that day and what household chores must be done before bedtime, it is quite possible to forget that this is Thursday and your child is in the backseat.
In a nation with tens of millions of small children, an abandoned child every month or so is not really all that often, but I'm sure we agree that even once is too often. I like the idea of removing your shoes whenever your child is in the carseat as a reminder, except that I'm not sure driving with shoes is really safe.
For some people, leaving their cell phone in or under the car seat would solve the problem of reminding them they have the child. There's no way they'd go a half-hour without checking their phone.
