Originally Posted by
Often1
But, back to basics. There weren't 12 seats. OP specifically notes that the seats were sold. This means given to overbooked pax. So, the T-30 matters and it wouldn't have mattered if UA had held the flight 3 minutes or 30 minutes, once the inbounds missed the cutoff, their seats were gone.
All comes down to travel insurance.
I'm pretty sure Contract of Carriage (CoC) states that you have to be present and ready for boarding at the gate at T-15 minutes. At that point, UA is justified in giving away seats, not before then. (edit: seems I was wrong)
The T-30 mark is when domestic check-in closes, at which point GAs will often start doing standbys if the flight is underbooked. But, in this case, since it was an international flight, the relevant time is T-60 for check in closing.
Edit: hmm, interesting. From the CoC:
D)
1)
C)
All Passengers must be present at the loading gate for boarding at least 15 minutes prior to scheduled departure.
NOTE: If the Passenger’s itinerary includes an international destination, the international time limits in D) 2) below apply to all flights in the itinerary.
D)
2)
b)
All Passengers must be at the loading gate for boarding at least 30 minutes prior to scheduled departure. EXCEPTION: Within locations specified in D) 2) a) (ii) above, Passengers must be at the loading gate for boarding at least 60 minutes (1 hour) prior to scheduled departure.