Originally Posted by
Tedgrrrr
The only valid reason is marketing advantage, and it really is false advertising. I recall these old people from North Carolina who were completely confused coming back from SYD and being completely befuddled that they had to leave the plane, grab their bags, go through customs, check bags in again and reboard. They were fully aware of the "stop" but they thought they'd stay on board sleeping while passengers got on/off. It really is false advertising but AC apparently thinks it's a big marketing advantage.
AC could do a sterile transit at YVR without requiring those YYZ passengers to go through CBSA formalities at YVR if it wants but it just couldn't be bothered with it.
In the old days, YYZ passengers on CP8 from HKG and CP4 from NRT did not have to clear immigration and customs at YVR. They cleared when they arrived in YYZ.