It wasn't the Maersk Kensington and in any case it wasn't a U.S. flagged vessel.
Section 311
(a) The Government of the United States has full authority and responsibility for the security and defense matter in or relating to the Republic of the Marshall Islands.
(b) this authority and responsibility includes:
(1) the obligation to defend the Republic of the Marshall Islands and its people from attack or threats thereof as the United States and its citizens are defended;
(2) the option to foreclose access to or use of the Republic of the Marshall Islands by military personnel or for the military purposes of any third country; and
(3) the option to establish and use military areas and facilities in the Republic of the Marshall Islands, subject to the terms of the separate agreements referred to in sections 321 and 323.
(c) The Government of the United States confirms that it shall act in accordance with the principles of international law and the Charter of the United Nations in the exercise of this authority and responsibility.
And your point is? Are you saying that the U.S. has the responsibility to protect all of the hundreds of ships that choose to register in the Marshall Islands for tax reasons?