Indeed. Although there is the case of owners of rights invited to come forward to receive their due, if it wasn't possible to contact them before. You probably need a proof that you made an effort though.

It's also true that anyone can go to a public library and read all the contents for free- the point is they can't further distribute them except in a highly processed form (i.e. they can distribute original products influenced by what they have read). Here the issue is the scale of both the "reading" part and of the "producing original work" part.