As ajGoes mentioned above, it works differently in different cultures. As another couple of examples: in Eastern Europe you won't get any smiles unless they're genuine. The norm is an unemotional face, but if you see a smile - it's genuine.
In Japan - a smile is an indication that the person smiling is nervous/not at ease.
In middle eastern cultures, people will smile right and left at you but these smiles will not be genuine and you will never know what the person is thinking. (Just think of a shopkeeper at a souk/bazaar smiling before he cleans you out during your negotiation with him).

When a traveller throws his/her American standards onto every culture they visit - it just goes to show how culturally insensitive they are