From Stanford University Libraries' Copyright & Fair Use article "Public Domain Trouble Spots":
While it’s true that no strings are attached to using public domain materials, you should be aware of certain potholes on the public domain highway, as described below.
Works such as movies or sound recordings may contain many underlying works, such as musical sound tracks, painted illustrations, or other works. There has been a disturbing trend by some copyright owners to assert protection in an element of a public domain work. For example, the film It’s a Wonderful Life fell into the public domain because of a failure to renew copyright. For years, anyone was free to copy and sell the movie on videotape. However, a production company recently acquired rights to the musical sound track that is used in the movie. That sound track is not in the public domain. The copyright owner of the sound track can now prevent anyone from copying the music, thereby effectively stopping anyone from copying the film (unless the sound track is removed). Multilayered works can create confusion when trying to determine public domain status.
Usually you don’t have to be concerned with this type of legal maneuver as it is only used in connection with popular and older multilayered works such as classic films. It would be difficult, if not impossible, to apply this procedure to a public domain book or painting.
A movie musical containing songs by Cole Porter is in the public domain because of a failure to renew copyright. However, the Cole Porter songs were renewed in time, so they are still protected by copyright and cannot be reproduced without permission. Therefore, you must obtain permission from the copyright owner of the Cole Porter songs in order to copy the public domain film. If you do not want to obtain permission from the owner, you must delete the songs from the film.
Public Domain Works That Are Modified
Modifications to a public domain work may be protected by copyright and cannot be used without permission. A famous example used in many copyright classes is the artist who paints an elaborate hat and mustache on the Mona Lisa. Even though anyone is free to copy the image of the Mona Lisa, the modified image (with mustache and hat) is protected under the artist’s copyright.
Color has been added to the black-and-white public domain film God’s Little Acre. This colorization process is copyrightable. Therefore, the colorized version of God’s Little Acre cannot be copied without permission.
Works Protected by Trademark Law
It is possible that a work may not be protected by copyright, but is still protected by trademark laws. Chapter 10 provides more information on trademarks.
The gold-colored top of the New York Life building is in the public domain—anyone can photograph it. However, that image also functions as a trademark for the New York Life Insurance Company, and a competing company could not use the image if it would be likely to confuse life insurance consumers.
Plagiarism, Attribution, and the Public Domain
If you copy from a public domain writing, do you have to credit the author? The United States Supreme Court has answered, “No,” holding that there is no legal requirement to provide any attribution when public domain works are copied and placed into new works. (Dastar Corp. v. 20th Century Fox Film Corp., 123 S.Ct. 2041 (2003).)
However, just because there is no legal requirement to give credit to the creators of public domain works, doesn’t mean you don’t have to do it. When copying works from the public domain, be careful to avoid plagiarism.
Plagiarism occurs when someone poses as the originator of words he did not write, ideas he did not conceive, or facts he did not discover. Although you cannot be sued for plagiarizing a public domain work, doing so can result in serious professional and personal penalties. For example, in the case of college professors and journalists, it may result in termination; for students, it could lead to expulsion; if done by well-known historians, it can result in public humiliation.
Why is it so hard for IP laws to have it that when copyright expires, there should be way that it can be re-copyrighted or have something of it trademarked after the fact?
When it comes to copyright, this is how I would rule that it works:
-The duration of copyright is the life of the author+25 years
-Perpetual copyright of any kind isn't allowed and there are no exceptions
-Renewal is limited to 2 times and each has a 60-month lifespan.
-Any trademark of said work while it was copyrighted is also public domain and are to stay as such.
-The 2nd renewal has to be done at least 48 hours before public domain kicks in.
-If the life of the creator+25 years+2 extensions=70+ years, then it will become public domain even if the creator is still alive.
-There are no exceptions or circumstances (including a modification) after it becomes public domain where it can be re-copyrighted or re-trademarked.
-Any royalty fees they get that on the day or after the day that public domain kicks is voided.
-Once public domain, no entity can, even if wasn't the right holders, can claim copyright on it. That is, even if a publishing company were to publish/republish a modern version of a work that has long been in public domain like Tom Sawyer, the copyright symbol would have to be absent.