Seatbelts save your life from the many simple crashes where you would have simply smacked your head on the dash and been killed.
Clearly a crash that would tear appart a car or some head on smack ups, tossing people 100 feet onto bare streets...
The much more rare accidents those are, a seatbelt or lack of, would have been the least of your worries.
But out come the kids saying seatbelts kill. Because that one guy drowns or burns alive in his car and can't get out.
You just don't see the math apparently.