They are not "proving" they have the copyright. If a work is in the public domain you can use it without paying royalties. It is up to you to "prove" you have the copyright. None of us is lawyers, but from a reading of the U.S. copyright website you need to register your copyright before mounting a legal action. You may own the copyright but our novice interpretation of the U.S. copyright website leads us to believe without registration you cannot bring a successful legal action.
Originally Posted by marciofs