Isn't that the contract that requires a pilot to take a flight  

The contract says: "A normal schedule shift shall not exceed twelve (12) hours. This section will not relieve any Pilot from accepting any flight that may extend the shift beyond the scheduled twelve (12) hours as long as the flight can be conducted in accordance with the applicable FAR’s and Company duty time policies."

In a nutshell, pilots work a 12-hour shift unless they are called out on a flight. If called out, the total 'duty time' cannot exceed 14 hours. The Union protects pilot's ability to be back at their home base by 14 hours from the time s/he clocked in for duty.