To constitute the crime of piracy, the illegal acts must be committed for private ends. Consequently, an attack upon a merchant ship at sea for the purpose of achieving some criminal end, e.g., robbery, is an act of piracy as that term is currently defined in international law. Conversely, acts otherwise constituting piracy done for purely political motives, as in the case of insurgents not recognized as belligerents, are not piratical.
http://www.fas.org/irp/world/para/pirates.htm
Omar Bashiki