The CATSA screener may not have realized I was flying to the U.S. As Wally Bird noted, it is a small airport with a single waiting area for all gates. I had shown the guard at the entrance my Alaska/Horizon boarding pass to gain admission to the screening area but the screener didn't look at my boarding pass, and there were a couple of Westjet and Air Canada domestic flights departing at about the same time as my little turboprop to Seattle.

Of course, I didn't argue with him.

P.S.: Ummm, party_boy, with the arguable exception of Vancouver Island I don't think any part of British Columbia is "overseas" from Seattle, Washington. I mean, Kelowna isn't even on the other side of Puget Sound.