Said by a character with diplomatic immunity after a particularly interesting anti government statement: "We pretend they do not listen to us and they pretend they do not hear anything."
The possibility of unauthorized leaks of government intercepts attaches a cost to surveillance that cannot be ignored in any case.
Plus, if your communications have ramifications beyond those who are a party to the communications, then, even if you cannot be charged with crimes based on intercepts of your communications, the knowledge from the intercepts can be used by the government to change the world in ways that you consider harmful.