User blog comment:MTV/Captain Marvel/@comment-1212133-20100731032206

Superman isn't public domain because Action Comics #1 is still under copyright. Plus DC owns the Superman trademark so there really isn't much you can do with the cartoons to create a new work since most of the characters originated elsewhere. Same thing with reprinting the Green Hornet, Charlie Chan, or Saint comics all of these characters originated elsewhere so they are not public domain, but the original public domain stories could be re-printed, downloaded, and distributed freely. If you made a new version of the original stories you might be able to get away with it, but that's a big gamble.