The entire purpose of selling eBook readers (Nook, Kindle, etc.) is to sell eBooks, not readers. Like computer printers which are sold as conduits to printer ink sales, readers are intended to force most users to buy books from Amazon or B&N. There is no financial reason for vendors to enable Amazon eBooks on a Nook or vice versa because it would create direct competition and bring prices down.
On the other hand, it works to force both vendors to maintain larger inventories and offer more choices, lest the competitor wins on quality/quantity comparisons. I suspect most authors will not choose an exclusive for either vendor unless one runs away with the market.
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