Something like this can actually, and I kid you not, be role-played out. Simply role-play, with a proper reason, that your pals sit in the back of your truck. Maybe you're going for a cruise, you darn hill-billys, or there's simply not enough room for your crew that are all too cheap to buy a car. Also an efficient way of transporting several people, if you manage to avoid the chairs spewing out left and right.
And if you don't really think it's realistic, then I don't think you know anyone that owns a pick-up truck. I know several fellas, of which will remain unnamed, that will have their pals sit in the back, enjoy some beers or just go for a ride. It's illegal to go out on the streets doing it, but that hasn't stopped them. It's also risky, but definitely not unrealistic.
Since it's considered dangerous to sit in the back of a moving vehicle without wearing any form of safety equipment (i.e. a seatbelt), it may be considered illegal, and go against law 9.10, as the driver puts the lives of the guys in the back in danger. If any fall out and get injured/die, it is also against law 5.3, Manslaughter, as the driver will have killed someone due to negligence, especially since it's his responsibility that everyone in (or on) the vehicle are strapped down with safety equipment (most commonly a seatbelt).
So while I don't think it goes against any rules, except for 3.4 if you know the driver will be going to drive at any speed above 15-20 MPH, it is absolutely against the 2 laws that I have already mentioned.