From someone (not me) who's dealt with publishing excerpts in a book, at least one major publisher's lawyers have decided that the text of the speech was indeed

From someone (not me) who's dealt with publishing excerpts in a book, at least one major publisher's lawyers have decided that the text of the speech was indeed a "public performance" - given in a public place, and broadcast on public radio - and therefore not under copyright. The King family are apparently notoriously insistent on high royalties for any use of the estate's copyrighted works, but they seem to only have a firm lock on the video recordings. (Of course, this may be something that has yet to be tested in court, so Don't Try This At Home unless you have the money for a large lawsuit.)