AC must consider YYZ T1 offers enough of a sterile connection to market itself as a way of bypassing the US and its visa requirements for residents of many countries. Its international route expansion has in part predicated at scooping up customers from Europe/Asia who wanted to go to SAmerica (and vice versa) but without the hassle of getting the appropriate US transit visa. Canada B immigration is a cursory review of travel docs after which the passenger is channeled either to the US preclearance (if that's the destination) or back into the international wing. Obviously, AC would not permit boarding at the originating airport (of its flight) if the passenger did not have the proper visa for the final destination country, but they would not deny someone boarding if they were unable to get into Canada on their passport as the connection is sterile and not Canadian entry was intended.