>>I'm not sure how that would happen in practice. If my body is the projectile, they would have to be immediately in front of my vehicle as it slams into whatever it hit. My body is likely the least of their problems in that scenario.

Unfortunately, no :-( in crashes it's common for the person with a seatbelt to be killed by the body of the person without the seatbelt flying across inside the car like a cannonball. Bodies tend not to fly straight forward except for perfect head on collisions, and even in those cases the person sitting behind you without a seatbelt is going to kill you as they go through your seat. If you're alone in the vehicle I can maybe buy the argument that it doesn't matter, but even then there's plenty of examples of people being literally ejected out of the car and into harms way.