> She explained that that were she lived (Boston) you just couldn't safely wave at just anybody you passed.

I was born and raised in urban environments.

Let me translate this for rural (or fake rural aka suburbia) minded folks:

In cities people don't wave except to people they actually know/have seen several times because...

It's gauche, awkward, weird.

There are just too many people in cities and nobody can pretend their city is just one big village. People just go on with their lives and don't wave 1 million times per day. Waving is reserved to actual acquaintances.

In this specific case, it was most likely a bad neighborhood or someone with a heightened sense of fear due to reasons that cannot be clarified without knowing the person directly.