Yeah, it boils down to music copyright protection which can be pretty complex. Public domain music can be used commercially without restrictions or payment of royalties.
- Any piece of music published before 1/1/1923 is public domain.
- Any piece of music published between 1/1/1923 and 1/1/1964 that has not been copyrighted is public domain.
- Any piece of music published before 1/1/1978 has a 95 year copyright protection.
- Any piece of music published after 1/1/1978 is copyrighted for the life of the author / composer + 70 years.
For example, Michael Jackson died on June 25, 2009. His music will become public domain on June 26, 2079.