It might not be too hard to do in a way that satisfies Nintendo.
Imagine this:
In order to add an Amiibo to the app you need an Amiibo (obviously)
The Amiibo must first be linked to the account, which then saves the account information to the amiibo (and "locks" it)
Now the app can be used for all Amiibo data.
The Amiibo is useless until you delete the account or unlink the amiibo.
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