The betrayal of trust carries a heavy taboo.
Espionage, for the most part, involves finding a person who knows something or has something that you can induce them secretly to give to you. That almost always involves a betrayal of trust.
Let's say a Soviet exchange student back in the '70s would go back and tell the KGB about people and places and things that he'd seen and done and been involved with. This is not really espionage; there's no betrayal of trust.
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