At a certain narrow point, what's the use of carving out a particular class of people from a general "use seatbelts in cars" law? I suppose logically though so

At a certain narrow point, what's the use of carving out a particular class of people from a general "use seatbelts in cars" law? I suppose logically though someone with no dependents who signed some sort of waiver could be allowed, but it complicates enforcement measures quite a lot either way.