I started to write a reply about how whilst I agree largely derangedhermit there's an issue with there being limited competition when users are locked-in to particular stores by proprietary formats. However in writing this I realised there were a couple of things I don't know the answer to:
If someone wanted to compete with Amazon by offering mobi/KF8 format files would they be able to just go ahead and do so or do they need to license the format from Amazon who could then veto specific use (i.e. stop them competing)?
Assuming the formats are "open" in the sense that one can create and sell ebooks without Amazon's permission, do publishers have to agree to mobi/KF8 format exclusivity deals when they sell through Amazon?
Because I can buy an epub - of the same title - in various different stores and thus there's real competition. However unless I deDRM and convert then there's usually only Amazon selling Kindle-compatible formats.