Presumably that rule was made for safety reasons. Why would it be safe for flying cars to fly that low over urban areas when it isn't safe for regular airplanes to do so?
Take a look at the Jetson One https://www.jetsonaero.com/ It is definitely not a helicopter and incapable of the huge damage that helicopters do. The flight speed is up to only 63 mph. It's more like a small, flying scooter.