the other reason is that if canadian traffic can come into the US as domestic pre-cleared, then travelers are free to transfer within the US much more easily. t

the other reason is that if canadian traffic can come into the US as domestic pre-cleared, then travelers are free to transfer within the US much more easily. this makes things much easier for places that do not have direct connections to Canadian airports. (in the US, even if transiting to another country you are required to clear US customs.)