9.36.150 Carrying animals on outside of vehicle. No person driving a motor vehicle shall transport any animal in the back of the vehicle in a space intended for any load on the vehicle on a street unless the space is enclosed or has side and tail walls to a height of at least 46 inches extending vertically from the floor, or the animal is cross tethered to the vehicle, or is protected by a secured container or cage, in a manner which will prevent the animal from being thrown, falling or jumping from the vehicle. (CAC 9.36.150; AO No. 78-72; AO No. 82-191; AO No. 89-52)
According to history and animal husbandry, it prevents them from “rooting” in the ground for their food.