Tragedy is not usually due to a single cause. Multiple failures have to happen for a tragedy to occur. In this case, the mum left them unattended, and the manufacturer implemented a feature in a dangerous way. It should have been obvious in the design phase that this was a risk, to pets if nothing else. The solution is also well known, there have been groups lobbying to make occupant sensors standard for literally decades. That they implemented one without the other is arguably corporate negligence for nothing more than extremely minor cost savings.