Ari, while I agree that there are things that certainly could be considered more "dangerous" that are done everyday, common sense doesn't necessarily clarify the correlation between two laws such as why you're required to wear a seatbelt as an occupant of a vehicle, but it's ok that there are no restraints for motorcycles. If they're ok with you ejecting from a motorcycle, I suppose the same should apply for a vehicle. In Georgia, by reading Title Code 40-8-76.1 - my interpretation is that you aren't even required to have a gunners restraint system. The only time you're required to wear a restraint is as a front seat occupant or if you're in a category that requires them regardless such as children. I can't find anything about body parts being within the confines of a vehicle.
A properly working seatbelt is supposed to keep the passenger restrained in the seat so that they don't impact anything else in the vehicle. Vehicle safety or construction standards aside, a properly working seatbelt should keep you from getting a face full of windshield or dash board, so while "no one will be having a good day" is true - you can't tell me that someone properly restrained to a seat is in just as unfortunate of an an area as a guy standing in a turret. The gunner restraint system is nothing more than a tether - it keeps you from flying out of the vehicle, but it doesn't keep you from bouncing your ribs, back, head off the turret ring, gun mount, etc.
I'll let my kids ride in the top turret in the pasture next to my house, but I sure as heck won't do it on the road - child or adult.