Why? Because they can. ARM THE MERCHANT MARINERS.
These ships aren't "flagged" in the U.S., only a very few are from the U.S. Most of the crews are foreign and many speak no English, even the ones who come ashore a mile from my house. I have no idea what the rules are for carrying guns on the ships. I do know that many are robbed and sometimes killed while in the U.S. They are easy pickin's for punks because they rarely are available to testify against the robber if they are ever caught. Hopefully they have some kind of security for the ships, but I know they are sitting ducks just as soon as they step foot off the ship in Louisiana.