Not all the scores which are listed on CPDL actually reside on the CPDL server. Some links point to third-party websites which may have different copyright conditions attached. Check the type of license indicated and the site the edition references for more details.
The International Music Score Library works to gather all public domain music scores, in addition to the music scores of all contemporary composers (or their estates) who wish to release them to the public free of charge. Also known as the Petrucci Music Library.