Basically, the MPA wrote the registrar of the IMSLP (a public domain music hosting site), complaining that Rachmaninoff’s “The Bells” was under copyright and illegally available. The registrar, GoDaddy, immediately shut the IMSLP down.
It turns out that “The Bells” is in the public domain. The IMSLP makes it clear that public domain is tricky–there are over 180 different rules worldwide, but most are based around life plus 50, life plus 70, or the very confusing rules in the USA. The IMSLP, like the Choral Public Domain Library, takes care to examine and publish the state of public domain for every piece available.
Ultimately, it comes down to the point that music publishers do not want people to have access to public domain music, even if it is in the public domain or out of print. I cannot advocate this, but in my opinion, if a publisher is unwilling to keep something in print (particularly in this era when music can be stored digitally and published at need), it should instantly go into the public domain! Again, this is NOT law, and I do not practice this–but it is how I feel.
About a year ago, I worked with a nationally known choral conductor who publicly berated the CDPL as “flawed” and encouraged the use of purchased music, even for songs in the public domain. Meanwhile, he is the editor of a choral series under his name, and we’ve all found errors in scores.
Ultimately, I won’t be surprised when music publishers manage to change US Copyright law even further, protecting their income from composers who have been gone for years and years.