This is a recurrent question. Here are some elements to answer.
To the legal roots
Published works are protected by the Intellectual Property Code (see, in French, the Code de la propriété intellectuelle). The author owns rights on his work and is free to use them. The right to reproduce his work hence can be licensed to a publisher to be exploited (sold or rented), so as to be widespread and generate income for the author.
That’s why copying is generally forbidden. Some derogations are made for public schools or universities, with global conventions allowing them to copy limited parts on pedagogical purposes, while fees are funded to publishers. Another case is private copy: a protected work can be copied for private and personal use.
What about scores?
Music scores, for example, cannot be duplicated from a purchased copy when a group should prepare the same work, as it is the case in a choir or in a music course.
A work belongs to public domain 70 years after the author’s death—or last co-author’s death—, if it was made public. Some prolongations were negociated for war periodes, patriotic decorations or works that were unpublished before author’s death.
A work belonging to the public domain doesn’t mean copying the score is permitted: the graphic rights of the publisher can still be protected. This means that the work the publisher accomplished to setup the pages of a score, for instance, is still covered by copyright.