You wouldn't put your toddler in the bed of a truck! Why a Dog?

Its ILLEGAL to ride in back of a pick up truck under the age of 16, so why is it ok for a dog to be in the bed of a pick up truck freely and not in a secured down cage? That is so dangerous!! If you were to have someone rearend you the dog could also be severly injured or it could even be fatal, or even if you have to make a sudden stop or turn. Please help stop this. All animals regarless of size should be required to be a a secured down cage in the back of a pick up truck.