Higher import taxes (tariffs) on Canadian goods for Americans will increase the price, presumably reducing the amount of these goods that Americans will buy which hurts the Canadian producers. The only thing Canada can really do is hurt American producers in the same way, hence increase tariffs to reduce Canadian consumption of American goods and hurt American producers just as much (or preferably more). Hurting each other is a loss-loss strategy though, which explains why trade wars typically do not have winners only losers…

Not sure this is correct though. Of course all these companies are multinationals, but not all of these are American companies. For example Unilever is British and Nestle is Swiss (have not checked them all).