> (Boston) .... Why exactly would it be _dangerous_ to wave at someone in a friendly way?
It's not dangerous at all to do this. It's just considered odd & borderline impolite to do that. It's hard to explain to an outsider, but you see it brought up a ton on places like r/boston. The stereotype is that people in the northeast are "kind but not nice". By and large we don't engage in frivolity like greeting random people when walking around.
This is also true of the tube in London - if you talk to a stranger on the morning commute you may as well have peed in their backpack - the response is about the same.
And it can make sense. A lot of people are packed in small space and commuting. Not exactly an uplifting part of the day. I can understand it's considered polite to try to keep quiet.
What's funny is where I live now in Italy (outside of the big tourists areas) is the exact opposite. Any line or idle time you have with random people will become a conversation. It's almost weird to not have a conversation.