By law, you are required to use all of the seat belts the car came with. Since the car came with a shoulder belt, the law requires that you use it.
The Police Officer was simply enforcing the law as written. He doesn’t write the law, it is just his duty to enforce it. Just because other officers allowed you a pass doesn’t mean he had to.
You will lose in court if you decide to fight it because while a Police Officer has some judgement in enforcing the law, a Judge does not have that same latitude when it come to ruling on the enforcement when it is consistent with the law as written.
