I am not flight crew, but here are some comments on seat belts from scientific
point of view - it is much more than just cost.
1) It is not the speed that kills, but how quickly you change speed. You can
run into a wall and die at 20 mph. Of course, if you start at 250knots then it is
more likely you will change speed quite rapidly - but see 2
2) The type of accident in a car is quite different from that in a plane. Car accidents
are common and often involve rapid de-acceleration (you hit another car). The 3
point seat belt will often save your life, but it is also designed to keep you in the
car and in the right place to stop you hitting things - but also to provide restraint
to the limit of the car staying intact. Better seat belts in a normal
car are not worth installing since greater forces will result in complete destruction
of the car.
The physical plane can either survive intact in an accident, or break up; this
is very little different than a car. The forces required that result in the break up
of a plane are less than those required to break up a car, so the restraint for a
passenger need not be so comprehensive. Hence, the lap style seat belt is designed
to keep the passenger in the seat and not hurt other people and themselves to the
limit of the forces which will not destroy the plane. The question is therefore, why
not make stronger planes - this is difficult since this would increase weight. The
second option is therefore have a very strict criteria for use of planes - well
qualified pilots, controls on the plane operation, ATC, engineering and staff
to tell passengers to conform to rules that will keep them alive. This is
cheaper than building planes will survive a massive impact; even then we would
get the problem that the forces would be so massive that any form of restraint
would be pointless (lets not go there).
The lap belt is a sensible restraint for a plane if worn.
As to passengers not doing what they are told - since I am not flight crew I hopefully
I can get away with noting that it just improves the gene pool.
