On first reading the OP, I assumed they'd bought a stolen kindle, which was why it couldn't be registered; but it seems to be more complex than that.
On the point that Amazon make money by selling new kindles, I read (here, I think) that they sell the readers at a loss and make the money on the sale of ebooks. If that is the case, it wouldn't be in their interest to deliberately disable an ereader's ability to buy books.
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