Many songs which were collected from traditional singers were copywritten by the collectors, whether or not they made significant alterations. I believe the Lomaxes, the Warners and others have done this. The next question is whether others should profit by the efforts of those who were living memories or those who sought them out.
Many groups have used music without attribution, often out of ignorance, calling it public domain. Should Cecil Sharp's efforts be ignored, especially s he is reputed to have written some of the songs he "collected?"
Schooner Fare did it by mistake. When their attention was called to the error, they paid royalties and, I believe, corrected the error on later album releases.
Should the movies and their composers, with their vastly greater resources, not be held to the same standard?