No need to state the obvious
This is good advice. Out of town visitors might not be familiar with the New York state of mind, but people in the Big Apple, or in any big, metropolitan city, for that matter, value their solitude and their privacy. Perhaps it’s because that’s all they know, or that they’re used to it, whatever the reason – when visiting New York, keep this in mind – in New York, it’s considered impolite to maintain any eye contact with another person or acknowledge them in any way.
New Yorkers don’t need this reminder, so this is obviously meant for tourists. Perhaps in rural areas of the U.S, or in other countries in the world, it’s accepted to smile at the passers by, hold doors for them, or pay them a tipping of the hat, simply to acknowledge them, but in New York it’s the opposite – it’s considered rude, and an invasion of their privacy. New Yorkers, go figure.