Since when? The Internet was designed to be a robust way for Americans to communicate in the event of a nuclear attack.
I've also read about it in the book "Where Wizards Stay Up Late: The Origins of the Internet". In the prologue it mentions:
“Rumors had persisted for years that the ARPANET had been built to protect national security in the face of a nuclear attack. It was a myth that had gone unchallenged long enough to become widely accepted as fact.”
“Lately, the mainstream press had picked up the grim myth of a nuclear survival scenario and had presented it as an established truth. When Time magazine committed the error, Taylor wrote a letter to the editor, but the magazine didn’t print it. The effort to set the record straight was like chasing the wind; [Bob] Taylor was beginning to feel like a crank.”
Some other sources seem to confirm this, e.g. http://www.alphr.com/features/369490/top-ten-internet-histor...