The mission of the Navy is to maintain, train and equip combat-ready naval forces capable of winning wars, deterring aggression and maintaining freedom of the seas.
Exactly. The US cannot survive without international trade. We consume a lot of stuff we do not produce here. It's been that way since our founding.
Not to mention that unless ships are engaged in hostilities, generally ships are obligated to come to the aid of a nearby ship in distress. If you are a warship and aid involves sending some pirates to the sharks, so be it.