In the past it had less to do with seizing the vessels and more to do with keeping financial flows between organizations offering shipping services and oil hidden from the banking system. America could have easily seized any ship they wanted to during the sanctions over the past decade. They didnt because the sanctions are American constructs: they dont apply on the open seas where UNCLOS matters. America can still seize them, but the legality is murky and comes with a reputational cost.

Now with Hormuz closed, America needs every last oil barrel moving so the economy doesn’t grind to a halt. Remember, it’s a war of choice for the US. We don’t need Iran gone as much as we want low oil prices.

> the sanctions are American constructs: they dont apply on the open seas where UNCLOS matters

Technically correct. But the way these countries evade U.S. sanctions is by flying false or no flag. That, in turn, makes them vulnerable under UNCLOS's anti-piracy rules.