All is explained by the magic of competition -- which also explains why airlines hate competition (witness the ritual slaughter of Independence Air by UA and subsequent fare hike out of IAD).

With these flights it is easy to get off at your final Candian destination, since you have to claim your luggage in order to go through US customs. A simple matter to walk out of the airport instead of getting on the last leg. However, I think it's common courtesy to tell an agent you're not getting on the last leg (just say your plans have changed and you need to stay) so that the plane is not delayed trying to track you and your luggage down!