How this is in any way a "security and defense" matter
All countries defend their merchant ships on the high seas, or they become easy prey. American ships suffered many losses before its navy became strong enough to defend its ships--from Britain after the close of the Revolution, from France during the Quasi-War, from North Africa Moslems countries (ending in the Barbary wars), and from general piracy. For a little background, in one instance, you might study Allen's Our Navy and the Barbary Corsairs (1905).
The Marshall Islands have no defensive forces due the mutual agreement with America to defend and secure the Islands in all "matters in or relating to the Republic of the Marshall Islands." This includes their merchant fleet.
It's a flag of convenience. The ship has no connection to the Marshall Islands except on a piece of paper so to say that it's "their merchant fleet" is a gross exaggeration.