We have wide-ranging interests

"The main thing I want to emphasize is that I don't have an interest and the people at the NSA don't have an interest in doing anything other than making sure that (...) we can prevent a terrorist attack,"  said President Obama on August 9, 2013. That statement does not appear to be true.  The next batch of Snowden documents show that we have been spying on the European Union, United Nations, the International Atomic Energy Agency (IAEA) and nations around the world.  The spying goes beyond the world of terrorism. 

One NSA program, referred to as Barney, states that its main targets are "diplomatic establishment, counter-terrorism, foreign government and economic." Another program, Ramparts-T, is concerned with "penetration of hard targets at or near the leadership level" -- in other words: heads of state and their closest aides. We eavesdrop in 80 of our embassies and consulates around the world.

Back in the 20th century we did sign some conventions - the Convention on the Privileges and Immunities of the United Nations and the Vienna Convention on Diplomatic Relations of 1961 - which said that no espionage methods are to be used. I guess they don't count in the 21st century.