People are already out there in the streets breaking car windows because they see a dog inside the car. Do you really think people are going to put up with this kind of behavior? You're going to get pulled over quick, and not even by a cop. You'll get pulled over by a single mother named Debra who has ten cats at home already. She'll snatch that cat and bring it to a "good home."
That might be a dog, but let's go with cat. People are meaner to cats than they are to dogs. In fact, there are way more people who hate on cats than there are dogs. I don't think as many people would go out of their way to save a cat on a car roof than they would a dog. Those are just the sad facts for cats.