> The ships being seized are doing things like flying false flags, to try and trade with Cuba without paying tariffs.

Yes, they fly false flags to avoid triggering retaliatory tariffs. If country X sells oil to Cuba than country X's goods being imported to the the US will be subject to additional tariffs.

I can see how this wording makes it sound like the US is charging a tariff on the oil entering Cuba, but that is not the case. The tariff in that quote is referring to the tariffs the US is promising to place on counties that don't participate in the embargo.