While DRM is all about controlling the user open source software is about empowering the user. Open source software and DRM are a clash of principles.

They say: if Calibre were to support opening or viewing DRM files it could be trivially modified to be used as a tool for DRM removal which is illegal under today's laws. That's why Calibre developers don't like DRM. So if you have an Amazon Kindle and buy a EPUB book sold by Barnes and Nobles, you should know that if that e-book has DRM you will not be able to read it on your Kindle. People are often surprised that an e-book they have bought in a particular format cannot be converted to another format if the e-book has DRM. These restrictions prevent you from reformating the e-book to your liking, including making stylistic changes like adjusting the font sizes, although there is software that empowers you to do such things for non DRM books. Often people who buy books with DRM are unaware of the extent of these restrictions. DRM limits what you can do with e-books you have 'bought'. When you buy an e-book with DRM you don't really own it but just have a permission to use it in a manner dictated to you by the seller. DRM (Digital rights management) is a technology for access control that is used by ebook publishers and inhibits uses of ebooks that were not desired or foreseen by the content provider.

Calibre plugin to remove kindle drm