Belle Starr's connection to this song is folkie folklore and, so far as anyone knows, is based solely on N. Howard Thorp's comment in 1921 that it was:

"Written about 1878 [by "Belle Star [sic], Indian Territory"]. Song has been expurgated by me. The author was a member of a notorious gang of outlaws, but a very big-hearted woman. I knew her well."

Belle Starr: 1848-1889.

N. Howard Thorp: 1867-1940.

Somebody wrote the song, and it might as well have been Belle Starr, but instead it could have been any of several thousand other people.

Whether Starr told Thorp she wrote the song or he heard it from somebody else is unknown.   

All we know for sure is that Thorp thought for one reason or another that she'd written it. Alan Lomax influentially repeated the attribution in 1960.