Why is 6 seats cost share acceptable safety wise, but 4 seats will be say a limit on drivers licence standard medicals. But I can drive a 12 seat road vehicle on a standard drivers licence and medical... That's where the arbitrary nature of these rules start to stand out. I'm allowed to jeopardize 12 people on the road, 4 people in flight for the same medical requirements or 6 people if i want to cost share, 2 people if i want to build my own small plane and so on....
There is no coherent reason. That's one of my points.