The main goals of the program was to allo travelers direct access to Laguardia Airport, Reagan National and other US airports from Canada that do not have Customs and Immigration facilities.
Same with the Bahamas and Bermuda, you pre clear customs and can fly to LGA
I flew to Montreal last Summer from JFK
and experienced the Pre-Clearance procedures, basically after check-in and after paying the airport user fee (stupid) you go through regular airport style screening, then you enter a room that has lines for customs and immigration clearance.
Canadian Police stand guard at the door, if there's a problem I think the US Customs people turn the matter over to the Canadian's.
Airports that participate with the US are YUL
, Bermuda, Nassau.
Im not sure about Freeport Bahamas, Ottawa, Vancouver, Edmonton, Halifax, Quebec City, Calgary.
Im sure Regina, Saskatoon, Winnipeg, London, Hamilton, Victoria etc have too few trans-border flights to warrant the expense.