This is a non issue, if an airport is only set to process so many inbound intl. flights, and it has already meet or exceeded that qty. then people have to wait. This used to happen at IAD all the time when JFK would be hit with fog…we had an Egypt Air 747-300 full sitting all day, and did not de-plane until late evening…I helped with customs…it sucked.
It sounded like there was a communication issue between what was going on in Portland with the aircraft and getting any guidance from Aeromexico SOC.
The issue probably wasn’t that they couldn’t have waited to clear customs at PDX then go on to SEA. The issue would be that then the crew would have timed out and the return flight would have to be canceled/postponed a day and then the aircraft would have to sit in SEA all night without arrangements to do so. They decided it was better to get the plane back to MEX before the crew timed out and stranded the a/c in the US.