I found the following explanation on http://www.bbc.co.uk/worldservice/le.../page51.shtml:
So for example, if we’re looking at the United States the country we would normally use the singular verb so for example – “The United States is a very big country.”
Is it possible that the word normally suggests that it is allowed to use are? Even if we are looking at the USA as a country? I'm just curious...
The following sentence is taken from BNC:
The United States are part of my dreams because I grew up with its music, fashion, Burger King!