Even slave owners of old knew of things like this rule of civility.

Read no Letters, Books, or Papers in Company but when there is a Necessity for the doing of it you must ask leave. Come not near the Books or Writings of Another so as to read them, unless desired, or give your opinion of them unask’d; also look not nigh when another is writing a Letter.

In sum: it is always possible to read things you shouldn’t. The error is the reader’s, not the writer’s.