I have been in touch with two well-respected legal scholars, Francis Boyle from the University of Illinois College of Law and Scott Horton, a military and constitutional law expert. Both agree that the US had legal justification to use lethal force against the “pirates.” Boyle said, “Technically, piracy is a felony under US law. And deadly force can be used against someone involved in the commission of an ongoing felony.”
However, the killing of the three pirates may, and probably will, escalate violence in the area.
John Robb suggests funding an open-source counter-insurgency using Somalia militias who raid the pirates on land as the cheap[est, most effective method, even if the militias would probably present their own problems too.