This is the working of a Canadian Internation al airport for arrival and departures:

Arrival:

• Domestic - head into terminal
• International, no differentiation between the US (transborder) and other countries - clear CBSA Canadian immigration. The exception is transiting to a US-bound flight with an issued boarding pass. However, you will be questioned why if you are arriving from the US and then departing to the US.

Departure (3 distinct areas)

• Domestic
• International
• US Transboarder with US CBP preclearance

In your case, you are international arrival and US departure. There is no place to wait for your departure flight without going through the formality.