We have no ethical or legal responsibilities in this. The Marshall Islands are a flag of convenience. They'll let literally anyone register in their country so long as they pay the fees, and companies are lining up to do so because of the tax advantages. There is no connection with this ship and the Marshall Islands except on a piece of paper. The ship is not owned by the Marshall Islands. There are no citizens of the Marshall Islands onboard. The ship was not seized in the Marshall Islands so they weren't invaded. There is absolutely no reason why the U.S. should intervene in this. None whatsoever.
There is absolutely no reason why the U.S. should intervene in this.
Whether you like it or not, the United States guarantees freedom to sail in international waters of all American flagged ships (their crews, passengers, cargoes, etc.). Not only ships owned by Americans or American corporation but all ships registered as American, whether not owned by foreigners or foreign companies or not.
By treaty, which both the U.S. and the Marshall Islands agreed to (and enacted into law), the U.S. has to protect all Marshall Island flagged ships just as the do American flagged ships.