It depends on the age of the child and the type of seat. Some car seats of certain age ranges are designed to be fitted to lap belted seats, but others aren't. Are the chairs normally forward facing during travel?

I would guess it's a case of risk management. A child in a car with a lap belt only would probably be far more vulnerable than a child in a motor home in a similar crash - where there would be much greater impact absorption and they'd be higher / further back.

To me, the acid test is whether you can fit your seat(s) to the vehicle and they can be held in tight. Some cars have so much padding in the bum area, that it's almost impossible to get a really secure fitting, no matter how much weight you put onto the car seat and how hard you pull the belt tight! If after ten minutes of fitting, the seats are still flapping around like a pair of curtains in the wind then you have your answer ;)