Somali Pirate Economics
wired.com
wired.com
According to Nick Nuttall of the United Nations Environmental Programme, "Somalia has been used as a dumping ground for hazardous waste starting in the early 1990s, and continuing through the civil war there," and "European companies found it to be very cheap to get rid of the waste, costing as little as $2.50 a tonne, where waste disposal costs in Europe are something like $1000 a tonne."
If my family started getting radiation sickness, that would add quite a bit of emotional justification for piracy...
Src: http://en.wikipedia.org/wiki/Piracy_in_Somalia#Sovereignty_a...
It's also interesting that piracy is still low enough of a cost that it doesn't justify rerouting ships, landing marines, or any of the other traditional responses to piracy.
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.
Presumably US flagged vessels are armed?
Naturally, US military vessels, including the Coast Guard, are armed.
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.