International law continues to allow for privately owned civilian ships to travel the seas armed. This is not a new development. Even the UN Convention on the Law of the Sea preserves that right.
Not that private companies must put themselves at the mercy of the United Nations, but still, it's significant that armed self-defense is still OK with the UN.
So private merchant vessels at risk of piracy aren't navigating undefended because of any international ban. Industry practice is to "avoid" traveling armed, according to a Washington Post op-ed, simply because that's been the policy of most companies. Satus quo.
Given the ineffectiveness of the world's navies in stopping piracy, and the over-stretched US Navy that advocates private sector initiatives, it looks like Blackwater is on the cutting edge again.