From Robert Paul Wolff:
But whereas it seems to make a very great deal of difference, for example, whether the Gypsy Girl that hangs in the Denon Wing in the Louvre is truly by Frans Hals, it makes no difference at all whether my copy of Hamlet is an authentic First Folio or a cheap paperback edition meant for schoolboys and girls. The words are the same [leaving aside scholarly debates about variants, etc.], and that is all that matters.... This makes music and literature purer art than painting and sculpture.
This seems like a category mistake. Perhaps the easy duplication makes for purer intellectual property, but not purer art.