It started with scores that were out of copyright but has expanded to those made available under Creative Commons license. Guo was sued in 2007 by a commercial publisher of scores in Europe and had to close for a time because he didn't have the resources to fight. His solution is ingenious. He set the website up in Canada where copyright is less onerous in a separate corporation to remove personal responsibility and disclaims local legal responsibility. He warns downloaders that they are responsible for complying with their local variants on copyright. The organization is now run largely by volunteers. It arranges low cost printing services in addition to free downloads.
The economics of this is that the old line music publishers are about to become largely technologically unemployed, as their business will be increasingly reduced to publishing current works or copyrightable corrected versions of those out of copyright. Wakins quotes both Guo's public service logic in promoting a much cheaper innovation and the defense of music publishers that their profits helped induce publishing new music. That excuse sounds pretty feeble.