Anti-subsidy rules aren't exactly a totally contrived reason for tariffs. If the American company is working without subsidy, then it's hard to compete on even footing when the Canadian competitor is underwritten by the state. The correct forum for litigating this question is the WTO.

However, this analysis is complicated by Boeing's close relationship with the USG, particularly the military. I've seen the argument that the juicy contracts they get for military aircraft and weapons systems constitute an indirect subsidy for civilian aircraft. To be honest, I lack the expertise to evaluate this objection, so I simply note it for completeness' sake.