Also note that there is a difference between the copyright of the speech text and the copyright on the film that CBS made of King giving the speech. Works that can be copyrighted include the text of a speech, or a song composition. A performance cannot be copyrighted. A recording of a performance can be copyrighted. A work must be "fixed in a tangible medium" to be copyrighted. So King's estate holds the copyright on one work (the text), and CBS holds the copyright on the second work (the film of the speech). However, since the film "contains" a performance of the speech, copyright law also says that CBS can't sell the speech without compensating King's estate. They can however make Fair Use of the film for news purposes.
This also means that if you attend a concert, you can record it for your own personal use. If you make the recording, you hold the copyright on that work (the recording).