Here's why:
* Copyright: The theme song was composed by Morton Stevens and copyrighted in 1950.
* Copyright Duration: Copyright for musical works lasts for the life of the author plus 70 years (in the United States). Morton Stevens died in 2000.
* Therefore, the copyright on the Superman theme song is still in effect.
However, it's important to note:
* The theme song has been re-used and re-arranged many times. There are many versions of the theme song, and some of these versions may be in the public domain.
* Public domain status is complex and varies by country. While the original theme song is likely not in the public domain in the United States, it might be in some other countries.
Always research the copyright status of any work before using it. You can usually find information on copyright status on websites like the Library of Congress or Copyright.gov.