[AnotherCat]

The most likely cause of the behavior experienced in Windows 8.x that I can think of, is that the following was accidently performed, or performed without understanding the consequences. I would be very surprised if it was something that Windows did maliciously.

I'll give the Control Panel route as to what may have happened as the OP refers to being a desktop only user:

Control Panel>Default Programs>Set your default programs>Calibre>OK.

This will associate all file types that Calibre can open with Calibre.

Note that this is different to the Control Panel>Default Programs>Accept a file type or protocol with a program option where one can set the associations individually. As far as I have seen the difference is often not fully understood.

The OP states "If you install it, should it not open epub files, for example?" from which one could infer that they may have been tempted to have set Calibre as a default program expecting that would just associate epub with it. Whereas, in fact, doing so will associate every file type Calibre can open with it.