Originally Posted by corporate-wage-slave

I think I know the answer to that one. Westjet used to have an extensive codeshare arrangement with BA, which ended in 2016. Before then BA.com used to give fares to all sorts of Canadian destinations. That ended but the interline ticketing arrangement continues to live on. So for example if we look at fare basis LLX0C1B4 for LHR to YHZ, with a base fare of £575 return, we see the following condition:

Transfers:
1 PERMITTED [TRANSFER] BETWEEN AA/BA/IB AND WS IN
CANADA IN EACH DIRECTION

and the same fare basis is used for allowing transfers on to AA services (though 2 are allowed in that situation) when it is a flight involving the USA.

The point being that Air Canada is in Star Alliance and flies out of LHR, so with little incentive to give good interline prices to a competitor.