Edit:I was certainly not clear about the "hand carry" rule for US origin plants, but I did hear a few Canadian hobbyists could not bring in(in car) Madagascar (origin)/wild Angreacoids without legal papers and few other incidents, such as Redland show story. So, not all the plants you purchase in US can be brought in Canada without papers, you can't bring all plants into US without some legal documents either.
To clarify.... Plants being carried into Canada without a permit must originate from the USA. To originate from the USA the plants must have been grown in the USA for a minimum of 12 months if they were first imported into the USA. The Madagascar plants you reference were simply traveling through the USA in transit so they did not qualify for the "no permit" option.
Also it makes a difference what state in the USA the plants originate from, example, plants from California require a phytosanitary permit and New York does not.