The problem with leaving it up to pilots to call fatigue is that certain "go to" pilots and operators can end up with an unfair competitive advantage.
If an operator has a pilot that they can push to fly on three hours of sleep, how do you think they are going to view the guy that (unbelievably!) requires eight hours immediately prior to working a fourteen hour day?
Very specifically dictating what does and does not constitute rest, levels the playing field.
Regarding the scheduled vs. unscheduled issue, 91K ops also comply with this definition of rest. Those are not scheduled ops, and we all know how Netjets is just barely scraping by.
