I'm not sure it's exactly a ringing endorsement of the Copyright Act of 1976 that Martin Luther King Jr. didn't take the time out of his civil rights campaign to work out the copyright status of everything he ever said in the less than half a decade between the time he gave this speech and his untimely death.
This sure makes it sound to me like he was more concerned with the civil rights movement of the 1960s than with the copyright status of his speech in 2025.
EDIT: Come on, downvoters — do you really mean to suggest that King would have wanted a civil rights memorial to have to pay out his family before it could celebrate his cause?