Almost 6 years before I was born, Martin Luther King, Jr. gave his famous "I have a dream" speech. The speech was copyrighted by King and his estate has sued companies such as CBS News for copyright infringement. The speech will not enter the public domain until 2038 when I am 69 years old.
In a Washington Post article attorney Josh Schiller describes the legal history of King's speech and reports that the lawsuits filed against media companies were resolved through "undisclosed settlements."
To understand why this speech isn't "free" we must take a look at copyright law.
In 1790 America adopted its first copyright law. The law allowed an author to copyright their literature for 14 years and renew their copyright for an additional 14 years. So copyrighted works would enter public domain no later than 28 years.
When I took a media law class in college, we studied copyright law and learned about the Copyright Act of 1976 which extended copyright to a maximum of 75 years. After my graduation, Congress passed the Copyright Term Extension Act of 1998 which added an additional 20 years to personal copyright and copyright protection for corporate works were extended to a maximum of 120 years.
These copyright issues are explained in the United States Copyright Office document Copyright Basics which is available free online.