Seriously, they don't want to start anything with the U.S. Navy. Seriously. Any skipper of anything larger than a mike boat who didn't run on them with guns blazing would be hooted out of his commission.
Our Destroyers are no slouches, and their 5 inch gun is plenty, not to mention everything else they have onboard.
Navies and pirates are natural enemies. It must be every skipper's dream to have someone he can go after without a care in the world for diplomatic implications.