This flight is an international flight. The passengers must be held until FIS agents arrive if none are available.
Oak is an International airport. They have FIS agents. If they were waiting for customs agents, I'm sure AeroMexico would have announced that loud and clear.
FIS is not open 24/7. And in this particular case, OAK appears to have had a FIS staffing issue due to the government shutdown.
You're making an assumption that is not supported by what we know from the article:
Alameda County Sheriff’s spokesman Ray Kelly said: "The decision to keep passengers on the plane falls to the airline
Another point in the article states that there were two immigration officers were working, (this was in the middle of a weekday.)
It sure looks like Aeromexico was more interested in getting their plane re-positioned and avoiding transportation fees for the passengers than they were about the well being of their passengers. The article did say that "only two immigration officers were working because of the federal government shutdown, slowing down the customs process
". But the article didn't pin the holding passengers captive on that fact and I think that would have made great news if it could have been written that way.
If I was on the plane there probably would have been three people arrested. Keeping people prisoner on a plane sitting on a tarmac is not cool.