Re: "already been blamed for one fatal crash and three others involving serious injuries"
"No they were not, air bags should not go off if you do not have a seat belt on, they will cause greater injury, as in death, if you don't have a seat belt on. They are to reduce neck and head injury caused when wearing a seat belt."
Er, it depends on where you are.
Across Europe and vast swathes of the civilised world it's normal (and a legal requirement) to wear a seat belt. The thing that actually saves your life is the explosive charge in the seatbelt pre-tensioner that goes off as the shunt starts. This hauls you back rigidly into your seat, and allows the crumple zones of the car to absorb your kinetic energy as the passenger cell (and the now firmly restrained passengers) decelerates. The airbag (which is actually quite small) simply goes off to prevent you face being mushed by the steering wheel or dashboard and to take some of the energy out of the head, reducing the whiplash caused by the body recoiling back into the seat (where the headrest takes out the remainder).
Without the belt and pre-tensioner you're simply a 40+mph missile moving forward through the passenger cell, not being slowed down by the crumple zones at all, injuring or killing everyone else you hit along the way.
In the USA, where weirdly there's states where you don't have to wear a seat belt, the airbag takes on the primary job of decelerating the passengers. Consequently they have to be much bigger, needing to fill the space between the entirety of the passenger's body and the dashboard / steering wheel. They go off as the shunt starts, aiming to cushion the blow as the passenger's unrestrained body hurtles inexorably towards the dashboard and steering wheel. Clearly this is an inferior solution, is of no help whatsoever in a side-swipe, and actually leads to people not sat normally in their seat (leaning forward, for example) being killed by the airbags going off in quite minor shunts. An airbag is quite capable of decapitating someone. Nasty.
Why Is it Law To Wear A Seat Belt
This is because an unrestrained body inside a car involved in a high speed shunt can be lethal to other passengers in a car. For example, if you're a passenger in the back seat not wearing a seat belt, you will likely kill the driver in the front if the car is in a fast enough shunt. It's also not inconceivable that an unbelted passenger in one car kills people in the other in a head on collision as they fly through two windscreens. I turf out passengers who refuse to wear their belt; I don't want their stupidity to kill me if some other idiot decides to drive into my car at high speed.
The extra cost of your lengthier rehabilitation courtesy of the NHS subsequent to your more severe injuries in the less likely circumstances of you surviving such a crash is a secondary consideration.
This is also related to the reason why air passengers are told to put things away during landing and take off and put their seatbelts on. Things like laptops, phones, books, etc. all become 150mph+ missiles hurtling through the cabin if the plane crashes. Your ability to get out of a burning aircraft is greatly enhanced if you've not been knocked unconscious by some other idiots laptop. Same reason why they're not keen for people to be standing up during taxiing (might be doing 40mph+), or on take-off / climb or landing / approach; passengers have no warning of how the aircraft will manoeuvre / collide next, and unseated / unbelted they're 80+kg of fuckwit selfish lard just waiting to become a deadly flying object within the cabin. You only have to look at the carnage caused by high altitude turbulence where unbelted passengers cause a lot of injuries to others as well as themselves.
In all transport an unbelted passenger is a prime example of an ignorant and selfish human being. It's notable that the only forms of transport where belts are not generally compulsory is ones that are slow (city buses, trams), or not normally exposed to rapid decelerations (trains, ships, submarines). Of course when a bus or train or ship does have a high speed crash the carnage is immense. For example a sailor unfortunately died of head injuries and 98 were injured when USS San Francisco collided head on with a sea mount at 30+mph.
Trains operate at high speed but in highly constrained environments (the rails), and generally the only thing that is a threat to them is another train, wheel failure or derailment. But even then modern designs are quite remarkable. The seating is designed to help keep passengers more or less in place should a train derail. They're also pretty good nowadays at not coming apart in crashes, saving a lot of life. However, you're still probably better off in a backwards facing seat in the middle of a carriage in the middle of the train.