Hi, Nick -
Licensing songs for performance is quite easy, but publishing them or altering them gets complicated. In publication of the Rise Again Songbook, we had to directly contact the holder of the copyright and obtain a license. If we couldn't located the holder of the copyright, we couldn't publish the song. I don't know how much of that is U.S. law, and how much was the policy of our publisher, Hal Leonard.
-Joe-