[ps67]

I don't think that somebody but Amazon could have the force of constraining their reader in walled gardens. And I am not sure even for Amazon

For example Kobo sells their ebook in Kepub for downloading in Kobo devices (or Kobo app) but they sell Epub eventually with ADE DRM to be read on other devices. And it can read DRMed books bought elsewhere trough ADE. And many other devices don't even need to pass trough ADE. This thread is about Nook but I don't know if nook sells also ADE DRMed books, may be convenient to do it also for them.

About Amazon: may they are tempted by walled gardens in fact You can't read any book bought from Amazon elsewhere without conversion if it comes in AZW3 or KFX (other than DRM issues). Only if it comes in mobi, the worst ebook format still in circulation, You can read it elsewhere.

And about reading on a Kindle something not purchased from them: You can send to them the book but You will read in the mobi format, while books can be converted into AZW3 and KFX only because of calibre. And the cover books signifies that sideloading is not something they encourage too much.

Amazon is getting closer and closer but I don't think they will be able to reach the situation:
1. read my books only on my devices or app: may be they could but I don't know if it is convenient for them;
2. don't read on my devices anything but Ebooks purchased from me: it seems difficult because they should prevent USB downloading also for their books.

But my impression is that apart a minority most reader simply buy a book, download it trough wi fii and read it on their Kindle or Kobo or Nook or what else so they are already in the walled garden and don't need to escape from it.