However even if it was free Amazon would have to be paying Adobe if they wanted their devices to be compatible with other stores and their books to be compatible with other devices. After all Apple use the ePub format for iBooks but their books can only be used in iBooks and iBooks can't read DRM ePub from elsewhere
Amazon owns Mobipocket, so they'd be silly to switch to something they would have to pay for. Especially if they are doing rather well. Speaking of Apple and prices, Amazon behaves as a classic competitor, pushing prices down to undercut the competition, which makes the publishers unhappy. Apple OTOH was pushing prices up with cooperation with publishers, securing the best deal for themselves.