Canada they exclude mainly because of bags. Unlike domestic, bags must fly with passenger on international. Adding to this, coming back you clear customs in Canada. Under US CBP rules you technically not in US till touchdown. (They can cancel preclearance anytime) Also an employee must deal with clearing your bags if they get seperated from you. Not necessarily huge inconvenience but its one more manual step.

Domestoc standby works great since bags can be voluntarily separated.