While the NY Vehicle and Traffic law does not require that adults in rear car seats wear a seat belt, physics does! Passengers in a vehicle who are not wearing a seat belt can become projectiles during a crash, causing them to be ejected through windows or injure other occupants of the vehicle.
And even though an unbelted rear seat passenger cannot be ticketed, there are still legal consequences. A jury may consider whether a seat belt could have prevented the injuries to a rear seat passenger when deciding of they are entitled to damages, and may deny them a recovery on that basis.
So in the immortal words of Sgt Phil: Let’s be careful out there!!