Actually, I'd say that's within certain parts of North America.
It varies quite a bit from region to region in the US. In the Northeast in general and the NYC area in particular, good service is marked by minimal interaction. Waiters, for example, are expected to be prompt, efficient, and silent.
In the Southeast, on the other hand, there's generally a more leisurely approach. A waiter or shopkeeper is expected to be "neighborly." To converse, establish a rapport, let things take place at a relaxed pace, etc.
This also varies depending on the type of place you're talking about. The more urban the area, the more packed together everyone is. The more packed together, the more privacy, personal space, and timeliness become issues. In that case, customer interaction becomes more and more limited (by the standards of the region).
Conversely, the more people are spread out, the more they look for a sense of community and interaction. Contact becomes more important, and so more interaction is expected.
Here in suburban NJ, things are more relaxed and open than in the big city, but still less so than I've found in many Southern cities.
In New York, everyone just wants the line to keep moving. Let people get done and get out as soon as possible so they can get on with their lives. You don't ask questions. You don't stop to talk. And, if you're the customer, you do everything humanly possible to make sure you have payment in hand before you get to the register.
In New Jersey, you can stop to smile at the checkout person and exchange a few superficial pleasantries. While they're busy scanning your items. Still, it's best not to hold the line up too much. Odds are, though, most people are just going to keep it quiet. Not always, but mostly.
Down in Florida, I know several cashiers by name and something of a rapport with one of the baggers (though I think he overheard and perhaps misunderstood a comment of mine a couple of years ago... he's been decidedly less warm and chatty of late). We usually ask after one of the people in the bakery department, and if she happens to be there, there's no escaping, even if we're in a hurry.
Driving up and down I95, I've encountered a whole spectrum, varying from town to town and state to state.
And that's just the East Coast of the US.
In short (a bit late for that, I know...): What's polite and expected vary quite a bit from place to place, inside North America or not. It depends on local history, climate, population density, and probably even more factors that I couldn't even begin to name.
