According to Creative Clearance:

Under the 1976 Copyright Act, all works of art, including trailers, are copyrighted as soon as they are created. Since the U.S. joined the Berne Convention (a copyright treaty) in 1988, no formalities (such as copyright registration with the Copyright Office or a copyright notice) are required to maintain one's copyright. However, in order to be able to make use of the various remedies found in the U.S. Copyright Act, such as infringement damages, injunctive relief, etc., it is important to register your work with the Copyright Office and place a copyright notice with the date on it. Therefore, trailers after 1976 will be copyrighted and will require permission for usage from the copyright holder whether or not they have a copyright notice or are registered with the Copyright Office.

Most of the trailers prior to 1976 were created as new works, which contained new material (such as" Coming Soon" etc.) as well as scenes from the films they were advertising. The trailers did not contain copyright notices nor were they registered in the Copyright Office or the Library of Congress. Consequently, the new material at the very least went into the public domain. Many of these trailers also contained material that appeared to be from the movie but was actually shot directly for the trailer. That material, since it did not contain a copyright notice, would also fall into the public domain.

The major argument has been that the scenes from the film itself were protected by the copyright on the complete film. However, one could argue that once you cut a clip from a film, it is a separate entity and without a complete and separate copyright and notice, it too becomes public domain by its publication. Because of this, most studio contracts have required licensees of clip material to copyright their productions so as to maintain the studio's copyright in the clip.