Amazon has the same reason to do that that Apple did when dropping DRM from the iTunes music store.

And no, Amazon's mission is not to serve their publishers, because their publishers aren't the ones giving them giant piles of money. Their publishers give them products to sell; their customers give them money for those products. Amazon's mission is to get as much from their customers as possible. Whether they can get more from their customers with or without DRM is a reasonable question, but the publishers only come into it if there's a belief that a significant number of publishers would leave.

That's not really true.

Apple faced two pressures which forced them to drop DRM from their music store: (1) Amazon MP3 who used lack of DRM as a point of differentiation over Apple and (2) the threat of regulatory action in Europe due to lack of interoperability.

Amazon today is in the same position in terms of ebooks as Apple was with music at the time - it uses DRM to lock customers into its hardware product, in turn driving further purchases towards its own store. This may or may not be a revenue-maximising strategy for them but it certainly looks like a market share-maximising strategy. Apple is also in a similar position again trying to drive purchases to its own store.

At the same time, regulatory action on competition has been more focused on contractual terms and price collusion/fixing than on interoperability; regulators will probably want to wait a while after sorting that mess out to observe whether they see competition acting effectively or not before addressing interoperability in ebooks.

For the same pressures and reasons to cause DRM to be dropped from ebooks, I think it will take an outside competitor (probably a new entrant) without a significant stake in hardware AND with substantial buying power to break this cycle. I can't think of an obvious candidate to do this today but perhaps I'm missing one.

No, their mission is to serve their customers, even at the expense of their publishers.

In a perfect world, that would be true. But this isn't a perfect world, and Amazon is a publicly traded company, so their mission is to make money for their shareholders. That _may_ mean doing what is best for the people buying goods & services from them, but if serving the publishers over the readers benefits shareholders of AMZN more that the other way round, then they are obligated to do that.