Lest we tangle legal and moral realms, the following distinctions should be observed. When the anonymous Davenport pirate-reporter stole the words of another author, he (or she) committed plagiarism by improperly claiming ownership of those words and not attributing them to the true source. That is an ethical failing, and a professional one.
The reporter also may have committed copyright infringement if he reproduced the words (as seems likely) without the permission of the other author or publishing source. That is a legal wrong. Had the reporter credited the source, he would not have been guilty of any ethical lapse, but he still might have been a copyright infringer if he had not obtained permission--unless the copying was a fair use.
In the 1920s (and even today), the copyright in a staff reporter's article would have been owned by the newspaper in the first instance. By the same token, it is the Davenport Democrat (and only incidentally the reporter) that would have been sued for copyright infringement, had somebody known or cared about the piracy. The reporter's legal trespass would have been imputed to the paper, and copyright infringement is a "strict liability" tort. It doesn't matter how pure may be the heart of the copier.