Derivative Classification

How is it exactly that a letter Jane Harman writes to the CIA manages to be classified? Harman doesn't work for the CIA, after all. Spencer Ackerman gets the answer: a legal called "derivative classification." This seems like an odd legal principle to me, but it seems well-established.

This article available online at:

http://www.theatlantic.com/politics/archive/2007/12/derivative-classification/47347/