If you know you're in honey then you act accordingly. If you think you aren't in honey then you're more likely to let your guard down and get into trouble.
For instance, an embassy with clear telephone and telegraph lines knows they're being listened to, and subsequently is very careful about what they transmit. An embassy who has bought Crypto AG (https://en.wikipedia.org/wiki/Crypto_AG ) equipment thinks they are secure and transmits information they would never dream of sending if they knew they were being listened to.
Sure, but that's orders of magnitude more effort, oversight, manpower and political risk involved than your typical "find a thousand people who have a paper trail saying they did X, pick the five hundred of those who you have an airtight case against, pick the hundred of those who juries will find least sympathetic, pick the 50 of those least likely to shoot back" type" operation that the fedcop enforcement agencies run fairly unilaterally every single day.
For instance, an embassy with clear telephone and telegraph lines knows they're being listened to, and subsequently is very careful about what they transmit. An embassy who has bought Crypto AG (https://en.wikipedia.org/wiki/Crypto_AG ) equipment thinks they are secure and transmits information they would never dream of sending if they knew they were being listened to.