Which seems a little strange in the context that they appear to have been the ones who decided to take this particular action as a way of slapping around Hachette.
Actually, these actions seem driven by them not having a contract rather than chosen by Amazon. Without a contract, Amazon can't be sure of filling pre-orders at the promised price. Without a contract, Amazon can't return unsold books. The result is that they can't allow pre-orders or order books they aren't sure they'll sell. It also may be that Amazon normally gets credit for warehousing books with limited sales.
This is what the business looks like when selling without a contract.
Without an actual look at the contract negotiations, it's hard to say who's to blame for the lack of a contract. Perhaps both are. Perhaps authors are to blame for not demanding better contracts (authors often make less for digital books than print books, even though publishers charge Amazon more for them). The only thing that we can say externally is that book prices are often ridiculous. There is no reason for the digital copy of a new bestseller to be more expensive than the print copy.