Because bitcoin is currently a medium of speculation, not a medium of exchange, the big retailers' participation in the whole bitcoin craze is not healthy at this point in time. Basically, bitcoin is NOT ready to be used in this manner. If and when major retailers like Amazon start taking bitcoins, it will crash the price. It's better that these retailers stay the f**k away from bitcoins, so it can go To Da Moon.
That makes no sense, and common sense suggests the opposite. Which is more valuable, a crypto-currency that you can spend on Amazon or an otherwise identical crypto-currency that you can't spend on Amazon? The only thing that will make Bitcoins valuable for something other than speculation is value as a means of exchange.