why isn't this subclassified as SF genre too

Because this is not a science fiction film. It surprises me the amount of people who think Ben is actually 'manipulating time'. It's a metaphor, as stated throughout the film itself, for freezing moments in order to appreciate the beauty in the world around you. Even on the back of the DVD, you will see that it says, 'Ben imagines he is freezing time,' which infers that it's not real.

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'I was very shocked when my son told me his boyfriend was a homosexual.'

I think "magical realism" more closely describes this than "science fiction" because it involves a singular non-real phenomenon in an otherwise conventional story, using it to advance the narrative rather than being the subject of the narrative. The narrative is the main character's breakup and new romance, the freezing of time is a device used by the character as kind of a way of exploring his thoughts and emotions.

He doesn't actually do much with frozen time besides sketch at the store and the phenomenon itself seems kind of isolated, at least in that it doesn't seem to have any way of changing any outcomes (other than the manager getting moved and hit with the drink container). He doesn't read about crashes or catastrophes occuring and we don't get caught up in the complexities of what freezing time would mean.

If the story had been focused on time freezing, it would be more like science fiction. It's not hard to see how the first 1/3 of this movie (until he discovers he can freeze time) could be the same but then become a very different film if it became focused on freezing time and capers that ensued because of it.