The reason we have an established stance for the Collector’s Edition is because upgrading from Standard to CE provides additional in-game rewards and the Collector’s Edition includes physical items that collectors are interested in having. A retail copy of the Collector’s Edition may not arrive in time for someone to join the expansion on launch day, so we accommodate this practice.
There’s really no point in purchasing a second copy of the standard edition if you already have upgraded your account digitally. If you acquired a separate physical key, it would not be redeemable on your World of Warcraft account.