Obviously misunderstanding of copyright and its purpose.

Incidentally, copyrights are listed for the following
Lonnie Donegan, Skiffle Group. Three entries, marked copyright Leadbelly.
Lonnie Donegan- About 20 recordings as pressings on different labels, at different times, in compilations, etc.- Composer listed as Leadbelly
Donegan, many more pressings- no composer listed.
Donegan, 3-4 pressings, listed as tradition.
Donegan- no pressings listed with Donegan as composer.
The above from Allmusic AMG search.
About 200 more copyright covers listed.

Following the Library of Congress recordings made at the Cummins Prison in Arkansas, Leadbelly recorded for RCA Victor in 1940 and Capitol in 1944, among others.
Donegan recorded his skiffle version in 1956.
Johnnie Cash (who also had a copyright version of the song) sang it at the Cummins Prison in 1969, where it was first recorded.
From the online Encyclopedia of Arkansas entry on Rock Island Line.