Why did U.S. — not Canadian — fighter jets respond to alleged hijacking of B.C. plane?

IIRC it's mainly used for search and rescue.

True, though it doesn't base F18s for this kind of mission.

CFB Comox is the RCAF's primary air defence installation on Canada's Pacific coast and serves as the home base for maritime patrol / anti-submarine aircraft and search and rescue (SAR) aircraft, both fixed-wing and rotary-wing .

NORAD was supposed to protect us, we just never could have guessed it would have been the US attacking us.

By who, Alaska? Anything else will be spotted hours before they show up or be so much more of a problem (like a fleet of submarines) that a base or too closer to shore is pointless.