When watching a movie, the way you use the computer is by looking at it as opposed to touching buttons.
Your comparison is horrendous.
And there is no excuse whatsoever for calibre, or most applications, to override user preferences for going to sleep.
In fact, offhand I can ONLY think of the specific instance of media players as something which should keep the computer awake (though I could theoretically be overlooking something).
...
Thank you for confirming for us all that calibre is
working as intended.