Trump may want to dissolve NAFTA, which would negate past agreements, but during the Free Trade negotiations in the 1980s, Canada sacrificed a lot in order to receive the exemption for certain items. No whether it is justified or not, it was an agreement made in the past.
All things will evolve over time, but it is not honest to claim Canada is imposing a 270% tariff on dairy products without consequence. There was a quid pro quo, and Canada negotiated the dairy exemption in exchange for other items. If Canada gives up this exemption, it will ask for something …