Originally Posted by
ashill
Perhaps off topic, but is there any reason US Customs can’t do that at US airports for connecting passengers?
They
could, but: prior to the NYC WTC terror attack, they used to offer transit facilities to international to international passengers (you can occasionally see unused or repurposed transit lounges when walking from international arrival gates to USCBP in airports like DFW), but the last vestiges today are international to International (ITI) baggage through-checking for many ITI passengers transiting MIA and DFW.
But all passengers transiting the USA must still process immigration under ITI and are deposited in the customs hall (without having to retrieve baggage and re-check it) to process security to get back airside to connect.
Canada isn’t the only country with airports with US Pre-clearance. The primary airports in Abu Dhabi, Aruba, Bahamas, Bermuda and Ireland also offer USCBP pre-clearance.