I remember installing calibre once, because I needed to open an .epub file I had. Upon opening it, it started to "classify" stuff and build a sort of catalog, to which I kill -9'd it immediately and uninstalled it for good measure. But mounting filesystems is way, way worse than anything I imagined it was about to do!
While the filesystem issue showed stubbornness, the fact that it tried to build a catalog is perfectly reasonable, given it is an ebook management software, which happens to include an epub reader.
You really can't blame a program to do what it says to do.
I don't blame the program, I blame myself for installing a management software when all I wanted was an ebook viewer (and definitely NOT a management software of any kind).
FYI: Calibre contains a program appropriately named "ebook-viewer" that allows you to view a file (like an epub) without all the library management stuff.
Calibre is much more than an eBook reader. It's more of an authoring / reading / management suite.
It can manage eBook readers' libraries either natively or via plugins. It can work with my Kobo eReader for example. I use Calibre to convert standard epubs to kepub (kobo enhanced epub) and push to device and extract highlights from books mainly. It's also a very nice ebook library so I can search and push/read/modify what I want.
Because it's not a book-reader, but a suit of tools for ebooks. Which includes managing them, receiving data from certain source, reading them, converting beetween different formats, automating certain managment-tasks, offering a web-interface as also desktop-interface, and finally also managing synchronisation with many many ebook-readers (for which the mounting is neccessary). And it BTW has a very elaborated plugin-system which allows to add even more that what it can do out of the box.