Agree that this is a very difficult question that will take some years to really even understand the question.

Here are a few claims that I would make:

* The answer shouldn't turn on general availability or rarity of the required artifact. If I choose to create a painting and not make copies, that shouldn't grant a license to others to mass produce. There are a lot of ways to frame this aspect, but if you believe rarity changes the dynamic, then you are defacto granting the large entity with sufficient capital greater rights than the small creator who may not be able to mass produce a unique item in high demand. Granted these scenarios don't apply to Nintendo, but they are accurate, imo.

* Whether the software is already on-disc (or in the main download package). This is a harder question. It's not hard to understand that even a small extra download increases the seller's server costs. If it is a separate download, a strong (arguably) argument could made for computer hacking charges for downloading software for which one doesn't have a valid license. It's a much harder argument to make that one shouldn't be allowed to access software already in possession. It's also pretty analogous to any other software if the add-on download is freely available but only functions with the code. That just looks like plain software piracy. So I'd argue it does matter, and like opening the hood of your car you should be able to tinker with any code legitimately in your possession.

* The hacking difficulty. That is, where on the spectrum of plaintext "cheat code" to cryptographically secure+ does the mechanism lie? One thread made the claim that these devices are just fancy cheat codes that should be passed around without consequence. Even more extreme on the spectrum would be an honor system where the software asks you whether you "cross-my-heart" have the corresponding Amiibo in your possession (check a box for ownership). Is it ok to lie to such a system since they made it so easy to do so?

(+ for the sake of argument, I'm defining "secure" as really hard to break and needing special tools and know-how and not possible on a large scale, not automatable, etc.)

* The seller's purpose behind the collectibles / relative value of the code vs. collectible itself. Should it matter whether the code is almost an afterthought and the seller is really just creating cool tchotchke's? The reverse could also be true -- they could sell small identical tokens solely as the means to monetize the features. Clearly, I don't think the answer can turn on such an arbitrary and difficult define metric. Just because it was cheaply made or an exquisite piece of artwork in and of itself is just irrelevant, that is the relative value of the code vs. object itself.

* Impact on others. If the NFC codes were each unique and a central server tracked check ins (not the case here, evidently), forging your own codes would adversely affect a 3rd party trying to 'legitimately' use the same token. This is an objectively bad outcome, although an argument could me made that this is Nintendo's problem for selling an item whose value they couldn't guarantee, not the problem of the guy who generated someone else's code. But, if you were to buy an Amiibo, clone it, and then resell then clearly you'd be defrauding the buyer. Lots of intricacies here. This is one of the hardest aspects in theory in my opinion.

By the way, this last impact on others argument is a variation of the situation Apple recently had to address with regard to iPhone unlockers. They would desolder the hard drive from a locked (either stolen or from a device that was inadvertently not unlocked before being sold, thrown out or broken and then repaired by a 3rd party) iDevice, change the serial number by a digit or two until they found one that wasn't locked. Re-solder the drive and re-sell. Problem was that new serial was often in-use by someone else. Either of those users could effectively prevent (re-)activation of each other's device, although this wouldn't be immediately obvious to either. There were a lot of complaints from legitimate buyers of brand new devices that their device was locked with an email address they didn't recognize. Apple's response was to turn off the system where one could verify whether a serial number was unlocked, making it harder to find legitimate serial numbers. (Please let me know if I got any of these details wrong, but I followed the issue pretty closely and believe them accurate.)