That it's a cultural phenomenon in the US and not in [other place x] does not make it immediately fake, and that some people may be faking it (I'll assume this is the case for argument) does not mean the phenomenon itself is fake.

I'm an American that smiles and greets strangers regularly. It's not fake when I do it, and I know it's not fake when many of my acquaintances do it.

I do know that this is not a natural/universal thing (though it took a trip to Japan for me to materially understand that). But just because it's not universal doesn't mean that, where it happens, it's backed by some kind of underhanded, guarded cynicism.

So have a nice day. ;)

Of course. Many Finnish people are like that too. If I can't think of a single reason myself why that unknown person might be smiling at me then I must conclude s/he's just pretending to be friendly and probably trying something, too. Thus, smiling must have a context: friends smile to each other and that's all right because they're already friends, and friends like each other.

I'm Finnish too and feel the same way. If someone smiles and acts too friendly when I don't know them well, I feel like they may be trying to manipulate me in some way. After living abroad for a while I am starting to see that it's just the default way to act, but being unsure when someone is genuinely enjoying my company vs. just trying to act polite still makes me feel uneasy.

When I lived in Canada (in Québec - I'm from France) it mightily pissed me off when the cashier at the bank always asked me "hi! how are you?" with a large fake smile everytime I came to deposit a cheque or something.

I understand it's just the way store keepers, cashiers, waiters and other customer-facing staff have to behave in North America, but, well, it's difficult not to see it as insincere or hypocritical when you're not used to it.