I really do think people get too hung up on being able to buy books from a reader's built in or sponsored book store. I realize the manufacturer would prefer you buy from their store -especially if they sell the reader hardware at a discount, but unless they make it both convenient and cost effective people are going to shop around...
Never in history has it been easier to shop around for nearly any product and over such a large geography. I don't buy electronic parts from American resellers anymore, unless convenient, instead I can often source parts in higher quantities for less money directly from the factories that produce them or an agent. The days of the middleman are numbered unless they can provide added value or services.