Hence the music you buy is protected by this law.
Hence you can not ask the author to get a spare cd. Since he's not obliged to give you one. Also everything you want to do with the music is determined by the law.
[1] i am talking about the law that protects original creations, not the one where you have to pay for a patent.
That's not really the question. The question is, now that I have this scratched CD, why is it wrong for me to hop on a torrent and replace my scratched bits?
Obligation to produce a new CD would indeed require an actual contract, but "new CD" is not the same as "replacing my music".
The real problem this exposes is that the law has a concept of "source of the bits", such that "bits from your CD player" are different from "bits from a torrent you downloaded", even if they are otherwise identical. Standard reference to the "color of bits" essay: http://ansuz.sooke.bc.ca/lawpoli/colour/2004061001.php ... and I'll basically defer the rest of this comment to that essay since I doubt I can improve on it here.
You can totally ask an author for a spare CD; however, he's not obliged to give you one.
Also, you are mixing up contract law (licenses) with copyright law ("default" law which applies in the absence of a license or certain licensing terms).