You overlooked something important -- back then, publishers were liable for what they published, including letters to the editor.
So, if someone (say, Great Grandpappy McMurtry) slandered someone back them by calling him a moron or idiot or imbecile or liar in a letter to the editor, both the newspaper and Great Grandpappy McMurtry would be liable under the local laws of libel/slander/defamation. As a result, newspapers screened not only their own reporters but also the content of all the letters to the editor they published.
But not now.
The infamous federal Section 230 immunizes Yahoo, The Hoosier, Facebook and all other websites on which Great Grandpappy McMurtry posts. If slandered, your only choice is to track him down while the publisher goes scot-free.
There can't be any doubt that Section 230 contributes to the poison on the internet today.