WEAR YOUR SEATBELTS
They might be? The Doc clearly is, and seatbelts in back seats are usually just waist belts, and we don't have a clear view of the waists of the girls in the back seat.
Only waist belts are really only the case in center seats (and even then only in older cars by now), and with the way she's sitting between two seats there, clearly neither girl in the back is wearing theirs.