The G though in a normal landing and deceleration is nothing compared to a crash.
And it doesn't take much to give a relatively light mass enough of a difference in velocity to give it a marked energy differential when it wacks something, square of the velocity and all that. If the plane declerates at 5G its on a pivot so will act as a lever which will turn it into a rotational system. It only takes 18 joules of energy to break an arm.
But I suspect we are getting over complicated and that the seats have only been analysed in the arm rest down position for the relevent codes. So they put in the manuals that they need to be down which then ticks a box for approval.
Engineering wise you don't want things moving within a system as relatively quickly there are huge forces occur with relatively small differences in velocitys. Which is why air bags are so effective stopping the travel of the persons body before there is to much difference between the relative velocity of the persons head and the dash board.