Every time someone new gets in charge, there's a new push for seat belt usage so they can show how concerned they are for safety and to deflect any liability. Everyone signs off on it and pays lip service to it.

And then it eventually gets forgotten. Why? Because EMTs and Firefighters in the back of the rig are actually DOING SHIT while riding to a call or to the hospital. Fire and EMS officers that never ride (or haven't ridden in ages) pretend that's not a thing. Oh yeah, just wear your seat belt while putting on gear, finding the map book, finding the knox box keys, pulling out bags and checking your equipment. And if you are in the back with a patient, I guarantee you are not just sitting there in your seat belt, there's interventions to be done.

I agree that you shouldn't just sit there unbelted when there is no actual emergency. But there's an institutional blindness in the upper echelons of Fire/EMS as to what actually happens during a run, they actually think you just sit there holding the mike and look out the window.