I think a lot depends on how much experience each pilot has. Each situation is different and the FAA looks at incidents on a case by case basis. If you're a brand new private pilot flying as a passenger with a 2000 hour CFI at the controls, and the CFI busts somebody's airspace, I think the FAA would look at it differently than if a private pilot were flying and an experienced CFI was technically a passenger.
I once heard of an ATP rated pilot that got violated when he was in the back seat on a XC, riding along with two commercial students in the front seats. Supposedly since he had more experience the FAA held him responsible for the other pilots busting a reg. Who knows if that's a true story though. Sounds somewhat plausible, but could definately be urban legend that stems from some pilots' distaste for the FAA.
All I can say is that I'm careful with things like that. I had a couple students pass checkrides this summer. When we got in the plane to leave the examiner's airport I said, "Congratulations, have fun. You're PIC now, I'm not going to give you dual for this flight. I'm going to kick back and chill as a passenger for once. Try not to kill us." I let them do everything without saying a word, but you better believe I stayed on my toes just as much as I do when I'm actually instructing.