I'm going to use a specific example... which has always been upheld to be true here.
If I'm at home, and a neighbor comes to visit, and they ring the doorbell/knock, it has been stated that I am under no obligation to answer it if I do not choose to, and I am not rude to not choose to. That they can see me through the window is irrelevant. That they know I heard them is irrelevant. That I looked directly at them through the window and made eye contact is irrelevant. As it's been repeatedly posited, just because I am greeted by invitation to interact (by knocking or doorbell), I am under absolutely no obligation to respond in kind. (If I'm misrepresenting this claim, I'd like to know... because I've read this time and time again.)
If that example above is rude, then it flies in the face of every unexpected-visitor-at-the-door thread I've ever read here. If that above example is not rude, then I don't see how being unexpectedly greeted on the street and not responding is.
I know that example isn't relevant to you, but the only difference is that in the first example, you're sitting on your couch, and in the second you're out and about. That's too slight a difference for me to be able to completely separate them.
An invitation to interact is an invitation to interact, whether it's a ringing phone, a knocking door, or a cheery "Good morning!" from someone I wouldn't know from Adam. My being outside of my house gives me no special obligation to consent to conversation, beyond that which I am either literally obligated to (if I worked retail, I'd have to talk to customers), or that I choose to. Not everyone is a social butterfly. Some leave the house purely to go from point A to point B.
That said, I *do* sometimes accept interaction, and respond in kind. But I do so by choice, not because it's the only polite choice available to me.