Ahhh, not sure if US and Canada share the same regulatory requirements. Chances are importing from Canada is prevented by US government regulations.
That one I did verify. The Mazda 6s sold in Canada meet the US safety and emissions requirements. The US Government is perfectly fine with you importing it. Its actually the OEMs that try to prevent it, but there are no gov't laws preventing it.
the US Gov't will charge you 2.5% at the border (slightly more complex at this, you don't pay tax on the first $400 and then the next 10%, so the effective rate is lower... on a $30,000 car you pay 2.5% of 26,640, or $666.). Which really equates to about 2.22%.
What you cannot import is a car not certified in the US under 25 years old. The Mazda 6 however IS certified for the US safety and emissions.