It follows then that the USA also wants to protect its industries, such as timber, oil and gas, and maple syrup, from Canadian producers that are happy to dump their stuff south of the border for cheaper than US competition.
I don't know about timber, but the US maple syrup industry is healthy, just unable to keep up with demand. I think that's a consequence of geography and the time investment required to get mature trees, not Canada undercutting.
A common point of political contention up here are various pipeline issues, precisely because it potentially permit access to more markets, improving the sale price. (However it would also raise the price of gas in Canada for the same reason, with all the implications that comes with that).
The USD and CAD are not 1:1, as the CAD is 'lower value', so of course stuff made in Canada will generally be less expensive. (The trade off is that imports are more expensive for Canadians.)