A tariff is a tax that a country imposes on goods entering its borders. A country can impose a tariff on any country, at any time, for whatever reason (unless they've signed free trade agreements obligating them to refrain from imposing tariffs).

> What would your reaction be if China imposed tariffs on US-Canadian border crossings and seized American ships over it?

Again, the ships in being sized were flying false flags, which is illegal. If American ships decided to take this criminal act, then China is justified in enforcing the law.