Under US law, where judge rulings become integral part of the law, all data and communications of any foreign citizen, even a parliamentarian, can be legally accessed by US security agencies without a warrant, provided that they are stored in the US (or by a US-based company).
While Ms. Jonsdottir’s specific situation is unique, many non-U.S. users of Twitter are rightfully unnerved. At least according to the magistrate and judge in Virginia, all of a users’ communications records can be subject to review by the U.S. government without a warrant because the users chose to use an online “cloud” service that stores data about them in the U.S.