Rescues, to my knowledge, don't transport dogs out of the country. Maybe breeders who call themselves rescuers, which sadly has become very common of late.
We had ONE yorkie go to Canada since 2006 and his adopter flew to the foster home to pick him up. We don't transport our dogs anywhere...and most of them are adults and fully vaccinated. ETA: I just realized you were talking about Canadian rescues sending to the US....I hope it does prevent some of that.
As to the CDC, I read just their overview of rabies and it makes total sense to me that they are doing what they are doing.
From their site:
CDC - Rabies around the World - Rabies
An additional figure is needed to complete the global picture of rabies. The best estimates of the impact of rabies on a country and the public health resources available within that country for rabies control are found in data for the number and distribution of cases of rabies in domestic animals. Despite evidence that control of dog rabies through programs of animal vaccination and elimination of stray dogs can reduce the incidence of human rabies, exposure to rabid dogs is still the cause of over 90% of human exposures to rabies and of over 99% of human deaths worldwide.
The cost of these programs prohibits their full implementation in much of the developing world, and in even the most prosperous countries the cost of an effective dog rabies control program is a drain on public health resources. The estimated annual expenditure for rabies prevention in the United States is over US$300 million, most of which is spent on dog vaccinations.
An annual turnover of approximately 25% in the dog population necessitates revaccination of millions of animals each year, and reintroduction of rabies through transport of infected animals from outside a controlled area is always a possibility should control programs lapse. Reservoirs of wildlife rabies, virtually unknown in Asia and tropical regions, are also potential sources of rabies infection for dogs in Europe and North America.
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As to your comments about trade between the countries, I doubt it is going to be some great problem because I cannot imagine it is a large trade for Canada. If it is true that it is such a large trade, then I applaud it because the numbers of homeless dogs in this country is reaching ridiculous numbers. If this would lower those numbers, it would be wonderful in my opinion even if that is not the reason for it.