pirates don't care about International Maritime Law nor Navy law. They are in the business of hijacking ships to earn profit. The laws are irrelevant.

Any ships that intend to travel long distance across the vast ocean should be armed with heavy fire power.

For instance, Somalia pirates killed people and hijack UN ships with ease since people are naive enough to be unarmed on open sea. The shipping lanes filled with cargo vessels that carry multi-million dollars goods. Why wouldn't pirates be tempted with hijacking? They can earn more money in 1 hijackking an oil tanker than their entire life time.