I can imagine there's plenty of "between the lines" stuff you learn as a CIA agent that, while not specifically classified, wouldn't be something you want going to other nations.
Decision making, for example. Someone internal to any organization will gain a sense of the way the organization will respond to certain situations. For something like national security, this can mean a contracted ex-NSA'r could provide another country with a forecast as to how the NSA will react internally to a certain stimuli.
It falls under "Tools, Techniques, and Procedures". It's the same reason anything published or shared is supposed to go through pre-pub even well after you've left one of the three letters.
