In most cultures, smiling at someone means, you know them personally. So, when you smile at a stranger, the stranger gets confused. They might think you are crazy. Also it could make them uncomfortable about how to respond. If they smile back, it means they know you, which is not true. Infact, if you smiled at a stranger of opposite gender, it could lead to other complications depending on who are with them, and they could get angry as well.
The phrase “low trust society” comes to mind.
No. It is lack of reason. Not long ago, if you went for morning jog in a village, you might be stopped and asked - I don't see a dog chasing you, why are you running? Same, if you give away some useful thing for free.