The question is… how deep does the rabbit hole go? Should all the linked records be duplicated all the way down the line? Or should just the first level links be duplicated? In a one-to-many relationship all the linked records would need to be physically duplicated and linked to the parent too. In a many-to-many relationship, the links themselves can just be copied.
Good question. You can think of one-to-many links as local/unique for that specific record and thus unclonable. And many-to-many links are more public, so of more interest while creating a new copy of the record.