Its not about the high seas. Its about entering ports, and the liability of having armed crewmen.
Many of the countries these ships go to do not allow armed merchant ships. Therefore, they would have to drop off the armed guards miles out, and still be liable to pirate attacks.
AND, you really do not want untrained, armed merchant marines getting all trigger happy on anyone coming near their ship. The insurance companies know that you are less likely to be harmed by pirates if you do not resist, and go the ransom route. It is all in the insurance tables.
I know it sounds really wimpy, but thats the way this thing has evolved. And, believe it or not, injuries to crew members that have been cooperative have been minimal.
I am not saying I wouldn’t want a sidearm if I were these guys, but this is the way its been.
That's not likely to continue after this, at least when Americans are captured by Mooselimb pirates.