It might be the rider who committed the avla, however, legally the driver is responsible to ensure that his passengers are seatbelted, so if he does not HE is committing an avla of another type, as well. He is responsible. it would be proper for the one who caused him to be nichshal, to pay the fine for him. If a parent lets a toddler cross the street without holding his hand, and he runs into traffic and gets hit by a car, whose fault is it?
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