Where do you live? If you're in CA I'm not sure what you are talking about because riding in the back of a truck is absolutely illegal, at least where I am it is. I also know that where my DD23 lives it's absolutely legal to ride in the back of a truck. We allow it but only when we are driving slowly down our families private roads.
Both the person I was responding to and I live in the wonderful state of AZ. I know it is not the only state that allows this(have relatives in a state that does also) but can't believe in this day and age it does.