Poor OP....he just posted wanting to know people's experience with this thing, and here we are debating the legalities.
Anyway, I have shit for memory, so going off what I read(and posted above), they wrote two different sections so they can(and do down here) enforce it whenever they want.
The first one(IMO) pretty much shoots you in the foot no matter what you wanna do, as it states that unless the manufacturer designed/intended for the specific area in/on the vehicle to be occupied by passengers, it's a no-go.
The second one states that the restraints have to be federally certified/specification(can't remember its exact wording).
Having said all that, the thought occurs to me that I'm not an expert on ANY laws, here or anywhere else, so all I can do is go off what I read and my own experience. So if someone is thinking of putting people(only adults hopefully) in the bed of their truck here in Kalifornistan, my only advice would be to consult the CHP and/or your local PD. I say both agencies because cities/counties have a lot of their own laws/ordinances that aren't necessarily also state/federal laws, and NOBODY knows the motor vehicle code better than the good 'ole CHP.
My personal experience with the law was when I was in my 20s(a LONG time ago), riding in my truck(I was pretty well lit so I was in the passenger seat, and two buds were in the bed). I decided to climb through the slider and engage them both in WWF, all the while there was a cop one car back in the next lane.
He of course lit us up, but in the end I was the only one that got a ticket for riding in the bed, as we were all cool and I asked him to solely ticket me since I caused us to be pulled over.
My second experience was when I did a ride-along(I used to own a Kawi dealer, and we serviced the vast majority of Kawi police bikes in SoCal, so I went on a couple ride along's with some of my friends that were PO's), and we pulled over a truck with two kids and a dog in the back.
I don't give this topic much thought -at all-, but as I said earlier, my position on it is that if you're an adult all the power to you.
The one thing I don't like is, kids and/or dogs in the bed. Dogs don't have a choice, and children must rely on adults to keep them safe.
Anyway, I apologize again to the OP for being part of his thread getting side-tracked.
I'll go away now