You'd think so. There must be legal issues with this approach, since you'd have to have deck-mounted machine guns to have much chance. Or maybe, as with everything else described in this article, deck-mounted machine guns wouldn't have enough cost-benefit.

Also, weapons on merchant ships can be taken by the pirates and used in future attacks. So unless the weapons allow the crew to consistently prevent hijackings, they're just increasing the pirates' revenue.

A substantial part of why foreign flagged merchant vessels get easy access to ports is that they are "known" to not be armed.

We wouldn't let a <pick a foreign country not too friendly with us> warship steam into Boston Harbor. Same country's oil tanker or cargo ship? Come on in.

A couple of shoulder-fire rocket launchers and some kalashnikovs wouldn't be hard to stash though, would they?

Presumably US flagged vessels are armed?

As a general rule, merchant marine vessels sailing under any flag are not armed. The US isn't "special" here.

Naturally, US military vessels, including the Coast Guard, are armed.

What of the right to bear arms then?

This is not a governmental restriction, but rather a commercial one.

Nothing prevents a shipping company from saying "In order to take a job on my private ship, you are barred from bringing weapons on-board."

Nothing prevents a country from saying "If your merchant vessel desires access to our ports, it shall not be armed."

The combination of those two (effects, not necessarily codified policies or laws) means that merchant vessels aren't armed.