This is untrue when private citizens are in conflict (legal or physical or whatever) and the dispute needs to be mediated or resolved. The 'root cause' may well be she said/he said and the legal system may well have need to resolve this by looking at private communications.

Strong emphasis on the legal system part, but you've completely dismissed that private communications frequently are required to address causes of issues and the word subpoena makes it rather clear that refusing to comply comes with penalty.