Okay, but there seems to be an agreement in epub world on how the DRM is done, or at least it was when I still had a Sony ereader. You got your AdobeID, set up your Adobe Digital Editions and your ereader with it and used the AdobeID when purchasing a book. If Adobe provides the encryption as a service or how it is done, I do not know, but the result is that regardless where you bought a book it is readable on your reader. Amazon's model is similar to Apple's app store, all (DRM-ed) content goes through them and there's no other way except not using DRM at all.
I believe that the Adobe Digital Editions process only acts as a gatekeeper service and it passes the licence request onto the retailer, who then validate the request and if it passes send the licence back to Adobe to unlock your book. Until the licence needs renewed it will just check everytime you 'access' the file with Adobe but if the licence needs renewed it will call back to the retailer again and do the process.
So its not really that different apart from the fact you have two potential fault points rather than one (If Adobe are down or the Retailers down (or pulls their service) you can't activate)
I can actually see Amazons system becoming the 'Steam' of eBooks more than the ePub setup simply because Adobe as the gatekeepers of DRM ePub simply have no motivation to become more than just gatekeepers. They don't have any hardware or content to justify them becoming more involved. Amazon on the otherhand have already shown a willingness to let other retailers into their eco system once they have a foothold (Amazon Payments and Marketplace for example) so I can see them in the next 18months licencing their format to retailers (Waterstones would be the obvious first step with their partnership with Amazon to sell Kindles) and maybe even to hardware manufacturers so other devices can also use Kindle books (after all they have put the Kindle Reading app on pretty much every tablet platform)