Somali Pirates

It's not an international crime.
Each case would be a crime by the pirates against the citizens on the ship, committed on the sovereign soil of the ship's registered country. So in the case of the kidnapped captain, it was as if the pirates came ashore and kidnapped him in Massachusetts and held him there.
Just like an illegal alien robbing a liquor store.
Now if the take the persons they kidnapped ashore it gets complicated.
IANAL, this is just how I have seen it explained.