Can you? I always have to show mine when flying from UK to US, and they have obviously seen my passport at least twice at that point. (Check-in counter in LHR plus gate at LHR) Or is this only when the transborder flight is your first flight? (Or when you are flying from anywhere in Canada)
But yes, I have also always found this odd, as you have already passed through immigration and customs, so it seems unnecessary.