In addition to original material, public domain is also free of any legal restrictions to use, perform, record, etc. There are reams of stories of venues and owners who shut down live music because ASCAP and their lawyers threatened to sue if not paid - even when the music performed was not copyrighted. ASCAP wins, because owners don't want to go to court, and have no idea they are being fooled. So there are almost no venues allowing live music, who aren't sending them a check. Its a dirty, underhanded practice.