Good question, and you'd be right that in that situation it wouldn't hold up to scrutiny. That's not what's going on, though. Instead, the tariff applies to trades American trade when it is determined that the other party is also trading with Cuba. The parent is correct; Mexico, or any other country, is free to trade with Cuba, but then it will be subject to American tariffs on American trade. It has to make the choice. There is certainly pressure, but it's on independent states to decide.