On one hand, it's as Kate says: too easy a fallback.
The other hand is slightly subjective. Consider how people smile. It's a continuum, from the passive face through something hinted at, to satisfaction, to maniacal. I suspect that a lot of the objection to smiling is that some people interpret it – by default – as the upper end of that scale; they don't notice the lower end of the scale. And there are also people who feel that others who are permanently a few notches above passive are somehow derranged or hiding something or… (Knowing that life is out to take the piss, I've got a permanent smirk on my face, and as such have been on the receiving end of such opinions.) So, effectively, this is an issue of lack of precision.
On one hand, it's as Kate says: too easy a fallback.
The other hand is slightly subjective. Consider how people smile. It's a continuum, from the passive face through something hinted at, to satisfaction, to maniacal. I suspect that a lot of the objection to smiling is that some people interpret it – by default – as the upper end of that scale; they don't notice the lower end of the scale. And there are also people who feel that others who are permanently a few notches above passive are somehow derranged or hiding something or… (Knowing that life is out to take the piss, I've got a permanent smirk on my face, and as such have been on the receiving end of such opinions.) So, effectively, this is an issue of lack of precision.