The Kindle can load its ebooks from anywhere.
In that case, the market dynamic is more like Android, or like music on the iPod prior to iTunes Plus: supporting only one digital restrictions management platform as well as DRM-free works from anywhere.
A lot of authors choose to use digital restrictions management so that they can sell more than one copy without having it be leaked to a mass infringement ring through an untraceable, judgment-proof member. The owner of an iPod can install DRM-free MP3 or M4A files from anywhere. But for nearly a decade after the iPod came out, the major record labels refused to sell DRM-free audio files over the Internet for fear of a leak to Napster or its successors (Gnutella, KaZaA, WinMX, and eDonkey2000), and iPod supported only the so-called FairPlay DRM platform used by iTunes. The owner of an Android device can install apps from anywhere (with "Unknown sources" turned on) with the APK file. But APK files lack DRM, so a lot of app developers publish their paid apps only through Google Play Store. And for the same reason, a lot of e-book authors publish their paid books only through DRM platforms. Can Kindle load e-books from any DRM platform, or does it support only Amazon's DRM platform?