About whether AC clearly states when booking that they might ask for the credit card when checking in:

Except, I do not recall that being mentioned anywhere when booking on aircanada.com. Of course I could have missed it. But OTOH, I do recall other airlines making it clear.

This said, I would also add that we only heard one side of the story. It seems likely that the AC fraud dept had flagged the file. But then, question is why. One might also remember that there was a time when their fraud dept was quite heavy handed, but then more recently after some bad publicity they seemed to have recognized that they were still bound by contract law. Anyway, at this point I would reserve my judgement.