To fall back on some sort of implied agreement implicitly admits that copyright is a matter of contract, not natural law for ones does not need to fall back on contract to protect natural rights. If a man steals your money, there is no need to appeal to an agreement to justify restitution. Restitution occurs because it was YOUR money. Only when you are dealing with those things to which you have no natural right must you appeal to contract. Now, historically, copyright has been handled differently than patents.
Many people accept copyrights while rejecting patents.
Copyright is said to protect style or the pattern of expression rather than the ideas expressed. The issue of duplication of style raises some interesting questions. For one thing, it is not unknown for poetry, especially short poems, to closely resemble each other. This would produce a similar pattern but not a duplicate one. If copyright would prevent me from doing this, then it is aimed not only at prohibiting exact duplications but at prohibiting similarities as well.