Oyster knew Amazon was going to end up trying this eventually. It's the obvious move. But Oyster also knows that the publishers do not like working with Amazon, which puts Oyster in a much better position to get good deals and get the publisher support necessary to have good books available.
So we'll see how well this works. If Amazon can't get the big 5 on board, but Oyster can, then I think they'll do just fine.