I grew up in Philly, and I live in Philly. Not Lower Merion, not Ardmore, Philly
. The rowhomes and neighborhood factories part.
It absolutely can be a safety issue, and it absolutely can be a boundaries issue.
As I have previously stated, I have had parents glare at me (the Icy Glare, *and* the Mama Bear Glare) for acknowledging their child. The way the environment of our society is these days, I would expect eventually people will be arrested for talking to a child they don't personally know. In some places, merely being in the same park as children (while being childless yourself) is suspicious activity.
Those of you who believe that your Ipod or Blackberry or random thoughts are more important than a strange human being
My personal time is just that, mine. Just as I am not required to answer a ringing telephone (a form of greeting) or a knock at the front door (a form of greeting), I am not required to answer a verbal greeting. If I'm using an MP3 player and someone greets me, *if* I notice it (and that's a huge if), I may nod to them, but I'm not going to answer verbally... if only because I'd be afraid of going "GOOD MORNING TO YOU!" Furthermore, if I am on a cell phone and someone greets me, I absolutely *will not* interrupt a conversation I am having to answer a random greeting. Why? Because that would be rude.
That said, I do answer greetings from strangers from time to time. But there is no requirement to. This is one of those "nice if you do it, but not rude if you don't" areas of etiquette.