greeting people

When I got back after my first visit of the US, people though I was way too polite - I would smile a lot and say "hi" even to clerks in the bank or somewhere else (and we do not greet those we do not know). Partly based on this politeness I became very good friends with a girl who got back from UK. She became more polite too. We were known as the most polite people in our department.
Greeting is a norm as well as "how are you" question. Although people usually do not expect any negative answer here, and I'm totally fine with any answer as long if it's true. If greeting is a norm, does that mean that I'm in trouble when someone does not say "Hi" at the beginning of the letter to me?
Personally, I try to put "hi" or some kind of a greeting in any message, unless it is not one out of several messages in conversation. And when I do not get greeting from a person I feel that maybe I've done something bad.

Or is that a form of a conversation where greeting can be omitted? What about SMS? is it polite not to say "Hi" there?

How are you?

As Wikipedia says 'politeness' is cultural thing. What is considered polite in one country can be rude in another.
My cultural background is the root to my confusion.
When people ask "How are you?" how come they expect to hear something like "good," "fine," "ok" or any other form of saying that things are ok? What if they are not?
Like today, a friend of mine is not really happy, he's miserable (as he said), but whenever someone calls he says "I'm good."
So how close you should you be to a person to stop lying about things?
Or this question does not have any meaning and used not to learn something, but just to "play" conversation according rules of the society? I guess that's how it is.

Still... if people do not really care why would they ask? Wouldn't that be more polite not to ask at all if you don't care? I guess that would be culturally unacceptable.