Originally Posted by
Lineman
Flying from Toronto to the USA, US Customs is done at YYZ. In theory, airports in the USA wouldn't need to have their own customs facilities if they only have international flights from YYZ.
In reality, though, this is not correct. Even though customs pre-clears passengers at the Canadian airport, the US airport on the other side must still have a Customs presence.
As a historical reference, AS was forced to cancel a nonstop YVR-SNA route because SNA didn't have a Customs office at the time:
http://www.flyertalk.com/forum/showthread.php?t=25773
Mike