First Person to Call it, Gets it
Duh. You can't just walk up to a car and casually hop up front. You wouldn't eat the last piece of pizza or eat the last Oreo without consulting the people you're sitting with (actually, you guys probably would, wouldn't you). We're civilized people, after all. That's why, after shotgun is taken, you then call non-bitch or left nut, right nut (or some variation of the rear seats).