Air carriers simply enforce the letter of Canadian law as the Canadian government advises the carriers. That is because the carriers are fined when an individual presents himself for admission to Canada without proper documents, even if Canadian authorities later admit the individual.

Put in its most basic terms, carriers have no discretion to waive the rules but border authorities do.

This is pretty much true the world over. If you can get yourself in front of the relevant border authority, you may make your case. The problem is getting that chance.