There are many level of espionage. The methods the government using right now is borderline idiocy.
When a person of interest know someone is listening and watching his every move and conversation, do you think he will be honest or will he introduce false information to misdirect the eavesdropper?
One goal of many of the programs that the NSA employed were to keep the program somewhat a secret, or at least keep the scope of the programs a secret. If someone does not know an attacker is listening to them (and thinks that an attacker is not listening in) then they would have no reason to provide misinformation.