Thomas M. Cooley's 1879 analysis of privacy of telegraphic correspondence (the "hampered by fears" link in the Guardian article) is fascinating: https://archive.org/details/jstor-3303981/
In brief, then, the doctrine that telegraph authorities may be
required to produce private messages, on the application of third
persons, is objected to, on the following grounds:--
1. That it defeats the policy of the law, which invites free com-
munication, and to the extent that it may discourage correspondence,
it operates as a restraint upon industry and enterprise, and, what is
of equal important, upon intimate social and family correspondence.
2. It violates the confidence which the law undertakes to render
secure, and makes the promise of the law a deception.
3. It seeks to reach a species of evidence which, from the very
course of the business, parties are interested to render blind and
misleading, and which, therefore, must often present us with error in
the guise of truth, under circumstances which preclude a discovery
of the deception.
4. It renders one of the most important convfeniences of modern life
susceptible at any moment of being used as an instrument of infinite
mischiefs in the community. It is not necessary to enumerate these
mischiefs. Any one can picture to his own mind what would be the
condition of things in any neighborhood, if its whole correspondence
were exposed to the public gaze. A single instance, in which the veil
of confidential secrecy is thrust aside, will introduce some of these
evils, but it will suggest the possibility that any moment all the others
may follow.
Inviolability of Telegraphic Correspondence.
Supreme Court of the United States.
Thomas Snell et al. v. The Atlantic Fire and Marine Insurance Company.
February 1, 1879