I remember when this project first came online. It reminded me of Dover scores, a collection of low-cost reprints of classical piano, orchestra and string quartet music. What Dover did was find old music editions that were out of copyright, scan them, and reprint them in a collected fashion. Everyone knew that these were not “critical” editions, and they didn’t include parts for orchestral pieces. But having large full size scores of all the Beethoven symphonies, or Mozart Piano Concertos, or string quartets, or Debussy orchestral music for a nominal cost was sheer joy to a young composer like me.
The IMSL is not much different from this, except it’s broader and user-driven. The purpose of copyright is to protect the rights of the composer and publisher. But when music is out-of-copyright, it should be open and available to whomever, wherever. I just don’t see this as being so different than what Dover started doing 30+ years ago.