A little rational and objective thought (always a novelty on pet cruelty threads) would suggest that people do leave their dogs in cars, but not with the intention of leaving them to bake to death.
More likely is that people don't stop to consider just how hot the interior of the car is going to get, and just how severe the change in temperature is going to be, and the dreadful effect it is going to have on their pet(s).
That sounds more plausible that the notion that dog owners take the dogs out simply to kill them.