It's more complicated than that. KFX was not encrypted differently than AZW, it's just a proprietary format that no one else supports (AZW being more or less MOBI with some tweaks). The DRM and the format get conflated because the same enthusiasts who want to strip DRM tend to want ebooks in an archivable, portable, standard format that was not achievable with KFX (no other ebook readers care to implement the kind of features it supports, and the way it works is antithetical to coverting it to the more conventional formats). You could still download and strip DRM in versions of Kindle for PC that pulled the KFX format. Only recently did it get to the point that versions of the app supported by the DeDRM plugins weren't allowed to download new books.