In Halacha there is no avairo to fail to wear a seatbelt except for the factor of Dina D’Malchusa.
The Chovos Halevavos in Shaar HaBitachon says that there is only a chiyuv hishtadlus for shmira for a hezek matzui. And getting into an accident where someone gets hurt is not a hezek matzui.
As far as Dina D’Malchusa, the obligation and fine is imposed by the state on the driver. He is the one who the secular law targets with the responsibility to ensure that his passengers are wearing seatbelts.
I don’t see how the person who was not wearing the seatbelt did an avlah. Against whom? The driver? How does he have a responsibility to the driver. The state mandates that the driver make a condition of being a passenger that they wear seatbelts. The only obligation that I see is on the driver to make a condition on the passenger that he wear the belt or leave. I agree with coffee addict.