Could it be that CX is looking at getting visas for the US based Canadians for reasons other than spite? Perhaps they could be looking at it from a morals point of view (unlikely I know) in that some of these people have been based for a number of years. Forcing them to return to HKG due to a change in US government requirements (if that's the case as Ali points out) is just wrong if there's something relatively simple that can be done.
The second possible (probable) reason is cost. It's cheaper to provide visas to the Canadians on US bases than to "repatriate" them to HKG or run the risk of them resigning over a forced return. Let's assume that all Canadians are forced to return to Hong Kong - are there enough US citizens to replace them on the US bases? If not, the net increase in expatriate benefits would far outweigh the cost a few visas. Let's assume that, having been forced to return to Hong Kong, all (or even some) Canadians resign - how are those pilots going to be replaced? I would hazard a guess that Tornado Ali's answer to both questions would be "Don't care" and "Don't care".
STP
PS. This wouldn't be the first time CX has sponsored pilots to work in a country other than their own - I think even US citizens needed a work visa when they came to Hong Kong.