Actually, isn't anyone who boarded an international flight required to go through customs to re-enter Canada? For example, once you enter the secure international departures gate area, you cannot return to any other part of the airport without going through customs. My understanding is that this is at least in part due to the possibility passengers can purchase duty free goods. If that's the way it is on the ground in Canada, I don't see why it wouldn't be the same case for an aircraft that has departed its initial airport but lands at another Canadian airport.