Hi rb.
You pose a good question.
I am guessing that in the realm of drivers and passengers, if chas veshalom there is an accident, the driver, whether in the same car or the car which caused the accident, would be responsible for any and all passengers.
In that light, if you had to choose who is responsible for seat belts, I think you would have to put the onus on the driver, even though we dont think of drivers enforcing seat belt use. I do, though, but only for front seat riders.
Halachically, I am guessing whatever the laws would be if someone comes into your home/business/property and they are careless/wreckless with their own safety, who would be responsible.
Interesting scenario/senahrio.