The offer is made every time you start up WoW, so it's not like players who decided to ignore the launch have that option anymore. The expansion costs $39.99 online, the same as in the the stores, preventing the need for pesky poisonous outdoor air to enter one's lungs.
The digital distribution question about WoW has been floating around since The Burning Crusade was announced. Guess it was nice for Blizzard to give the brick and mortar stores a chance before they totally cut them off and made their use in Burning Crusade distribution obsolete. Other than those who need the box to feel complete, what's the point of getting the Burning Crusade in a store now? Not to mention that this shows Blizzard will use a similar tactic in future expansions.