As other readers have said, I think children should never intentionally be left alone in cars - in fact, I believe it's illegal in many states.
However, the story you describe (and which happens several dozen times/year), is different. It is leaving a child in the car by accident. I think it's easy to judge and assume the person is horrible or sick, but I think that it actually can happen to anyone, especially sleep-deprived and stressed out parents. I have a dear friend (an amazing and very loving mother) who once accidentally left her sleeping child in the car - only for 15 minutes and he was fine - but she was shaken for days!
Another tip, besides leaving your briefcase/purse in the back seat, is to leave a stuffed animal or toy in the front seat with you to remind you that your child is in the back. I would also make sure that your daycare has a policy of calling if your child does not arrive at the normal time if you haven't yet called.