One of the Seattle attorneys suing Apple and five big book publishers for e-book “price fixing” explains why he did it (without saying whether or not Amazon’s involved in his move):

The new system was clearly not helpful to consumers, as it meant that they could no longer shop for a bargain amongst retailers. Instead, prices at each retailer would be identical. Alongside the elimination of competition between retailers over price, the agency model allowed, we believe, a 30 to 50 percent increase in the price of the ebooks.

Each publisher’s decision to sign an agreement with Apple was not illegal by itself. What would be illegal, however, would be the coordination of five of the largest publishers joining forces to thwart price competition. Given the nearly simultaneous timing of the actions of these five publishers, and the fact that their actions coincided with the launch of the iPad, we believe there was coordination.