It wasn’t a US flagged ship but it was registered in the Marchall Island which although getting independence from the US in 1986 still enjoys the “umbrella” of a US protectorate. So this is a stick in our eye.
If Obama had any stone at all he would have sunk that Iranian warship and then asked the Khomeni what he was going to do about it. Which would be beach and moan and chant death to America which he does everyday anyway.
The Marshall Islands are a flag of convenience where shipping companies will register their ships for tax purposes. Between Liberia, Panama, and the Marshall Islands close to all half to all ships are registered there. It has nothing to do with the U.S.