Iran seized a cargo ship that isn't American, which the US has accepted the obligation to protect.

I suspect that the Marshall Islands are one of the "flags of convenience". Most of the world's cargo ships fly the flag of one of the countries that impose little requirement on ships. This enables the owners of the ship to save money by skimping on safety requirements. Apparently the Marshall Islands flag carries protection by the US Navy as well.

I think the US should cancel arrangements for protection of any shipping unless it flies the US flag and, therefore, complies with US regulations for safety, pay rates, and so on.