Don't hold me to this, but I believe the pilots contract gives them a window where they are permitted to decline a flight before the FAA mandates the crew not rested. I don't know the details behind it though or how many hours it is.

Hypothetical Example: Pilots have 4 hours legal work time, but have the option to walk off while one hour legal.

Expected flight time is 2'55" - Crew must take the flight
Expected flight time is 3'30" - Crew has the option to take the flight or walk off
Expected flight time is 4'05" - Crew must not take the flight