Except your example is flawed. Business Class is physically a different seat and it is also a different service level. The exit rows have extra leg room due to regulatory requirements so are not a different "product" in the same way escape slides are not a different product to stairs. The passenger sitting in the exit row has to be willing and able to assist in an emergency. For that responsibility the airline is now charging the passenger for the privilege. It should be the other way around, the passenger should get a discount on their fare.