Originally Posted by
expert7700
With a US preclearance flight, the clearance is only valid at the original listed airport. They could have landed elsewhere in the US, but generally would be stuck waiting on the plane for conditions to improve at SFO.
The pre-clearance restriction only matters if letting pax off the plane. (CBP can choose to waive that, although I've been told it's pretty uncommon). Having to return all the way to Canada for an alternate would severely limit the US airports that AC could fly to. I have seen IAH-bound AC CR9s diverted to HOU, CLL, and LCH, for example, when IAH was experiencing WX issues. The aircraft will land and then go park at some remote part of the airport just to wait out the weather.
In the case of this flight, it's likely that (a) they weren't confident they'd be able to get to SFO at all today and/or (b) they were concerned the crew would time out by the time they'd be able to get in to SFO. Also possible there was MX or passenger medical issue or something, with the same concerns, but seems more likely that WX was the issue.