This was one of the things Jerry Siegel complained about in the '70s:
Also concerned in the 1947 case was Detective Comics, Inc. publishing "Lois Lane, Girl Reporter" and carrying the byline, "By Jerry and Joe", without any compensation being paid to us.
http://www.tcj.com/275/siegel1975.pdfPerhaps that's why the last names were left out of the byline - in order to avoid having to pay Jerry or Joe.