@Kovid: according to http://www.gnu.org/licenses/gpl-faq....prietarySystem
someone could distribute calibre along with a proprietary program as long as no claim is made that calibre is "part of" the proprietary program and they "communicate at arms length". This permits someone to ship a calibre "binary" separate from the proprietary app and for the proprietary app to use its command line interface.
Of course, the person must be clear about what they are doing to avoid implicit license inclusion. Quoting from the above link:
If people were to distribute GPL-covered software calling it “part of” a system that users know is partly proprietary, users might be uncertain of their rights regarding the GPL-covered software. But if they know that what they have received is a free program plus another program, side by side, their rights will be clear.