Tbh I always do this when there are few people in the bud.
My thought is that it is just an action that people doesn't like to see and not so much about hygiene , so when I see less people and Noone next to me, I will put.
If it is about hygiene then those parks benches etc need to be removed lo.
This sinkie case appear to be like my case, not crowded also.
The person just doesn't like to see and took the photo.
There was once I was plucking my toe nail with my right leg resting on my left thigh in a train that was not very crowded.
A sinkie man in his early 40s kind gave me the angered look and came to tell me I'm not supposed to do this in front of the public.
We almost end in fist fighting when I told him if u dislike u don't see.
I did not throw any toe nails on the floor nor did anything besides plucking my own toe.
So I warmed him to mind his own bs.