Well, the article actually explains how it was copyrighted. Basically the King estate sued CBS and the court determined they had a claim.
"Typically, a speech broadcast to a large audience on radio and television (and considered instrumental in historic political changes and ranked as the most important speech in 20th century American history) would seem to be a prime candidate for the public domain ..... the judge determined that the speech was a performance distributed to the news media and not the public, making it a “limited” as opposed to a “general” publication. That meant the speech, like other “performances” on CBS, was not in the public domain. That meant the King estate had the right to claim copyright"
"But the copyright dilemma began in December 1963, when King sued Mister Maestro, Inc., and Twentieth Century Fox Records Company to stop the unauthorized sale of records of the 17-minute oration."
There is a difference in going to court to protect it from being misused and sold without his consent to trying to lock it away so it can be a profit making item.
It is a case of going overboard. Initially they didn't care probably. But then they saw someone making money off it. So they didn't just go and stop that particular instance but went further and made sure nobody would be able to do it. They saw how money could be made from it, and decided they should be the ones profiting from it.
Seemed to start with King himself, according to the article. It seems like both him and his family donate the proceeds, however, which makes me feel better about the whole deal:
"
King himself donated proceeds from licensing the speech to fund the civil rights movement, and the King family has made similar pledges.
"
"Typically, a speech broadcast to a large audience on radio and television (and considered instrumental in historic political changes and ranked as the most important speech in 20th century American history) would seem to be a prime candidate for the public domain ..... the judge determined that the speech was a performance distributed to the news media and not the public, making it a “limited” as opposed to a “general” publication. That meant the speech, like other “performances” on CBS, was not in the public domain. That meant the King estate had the right to claim copyright"
This is relatively readable: http://www.ca11.uscourts.gov/opinions/ops/19989079.MAN.pdf