What makes this complicated is that Huawei does have a registered company in the US (with multiple offices), and therefore at least some part of Huawei ought to be subject to US laws. By making the choice to establish a US presence they ought to be willing to comply to the local regulations.
> If I pass a law that bans eating cornflakes, you then have breakfast and enter my country, am I justified in putting you under trial?
Yes, though the problem is that this example is trivial. If I pass a law that bans cyber attacks on my country's infrastructure, and you perform attacks in a country where it is legal and travel to my country, I'm well in the grounds to arrest you.
If cornflake eating were against the law, the solution would be to avoid traveling to that country in the same way that some people might not travel to North Korea for fear of being arrested unfairly.
It may be worth considering that laws are not always strictly confined to physical borders in what actions they can apply to. In this case, there are nuances that could be worth paying some attention to about Canadian laws and trade embargoes.
International law is never simple - although I suppose having the biggest stick might drive people to ignore certain nuances.