This is potentially (if we had a more ballsy POTUS) a "that was dumb, Iran" move, as under the Compact of Free Associatiom, the USA is responsible for the defense of the Marshall Islands (and their people/property) from foreign states.
ORA it's "smart Iran" knowing this, provoking on purpose, knowing we won't do jack.
But technically, for "do we defend it or not" it's more than a western ship, it's a "must be defended by the USA" ship...just like one flagged in the US.