A court found personal jurisdiction over an individual before the court, issued an order requiring the disclosure of information and then expected the person to comply with the order irrespective of where said information was stored. If you break American law in America, saying "the information is overseas" isn't--and shouldn't be--a valid excuse. Extending that to a third-party providers, when both the individual and provider are in America, makes sense.
Could this be abused? Sure. But the oral arguments show the Justices giving the question respectful concern.