Greece has confiscated Iranian ships, Iran retaliated by confiscating Greece ships. This is the least complicated geopolitical situation, that even an Israeli hasbara should be able to follow.
Sanctions are laws that are, and should be, applied to their own entities. If Iran had sold and shipped oil to a US company, America is free to confiscate the goods and penalize the company working in their jurisdiction.
US legal system doesn't apply to Iran. Meaning if an company exports goods from US to Iran, it is breaking American laws, but not Iranian laws. Greece has no authority what so ever to seize Irani oil tanker on behalf of US.
An example is Iran's own laws. Iran has made it illegal to import alcoholic beverages for public consumption. That means if a cargo vehicle of Heiniken is docked at the ports, it makes legal sense to confiscate it and penalize the importer. But it shouldn't be expected to confiscate Heiniken shipments all around the world.
The sanctions regime that exists currently makes no actual sense. USA is free to ban any import or export to Iran from it's territories, and make it illegal for it's citizens to trade with Iran. Going around the world to steal Iranian goods is basically criminal activity.
USA has normalized piracy and overextension of domestic laws. The Americans acts as a rogue state, their own legitimacy is that they hold the biggest stick and constantly use it. Greece got played into it and paid the price. Play stupid games, win stupid prizes.