The aggressive version of "buy American" being floated by Trump is a direct contravention of US obligations under NAFTA and give both Canada and affected Canadian companies (i.e. TransCanada in this case) avenues to seek redress under the terms of NAFTA.

It does make sense that both the Canadian Government and TransCanada are delaying making strong public protest and taking further legal action in the hope that a calm, steady but nonpublic spotlight approach will lead to the US quietly backing off (i.e. the assumption being that Trump's January 24th comment was simply bravado meant to probe and soften up Canadian resolve before actual negotiations begin and actual concrete demands made).

However, Canada can't afford to cave on this issue because this will only encourage the US to make other excessive demands in all areas of bilateral trade relations between Canada and the US.