I think the whole smiling thing comes down to the fact that people prefer people who smile, over people who don't.
Sure, I agree that it's stupid to do something like that just for other people, if they don't like our expression then that's their problem, but I think that kind of encouragement to smile is actually to benefit ourselves, not for other people. What I mean is, if people don't like your expression, it's not just their problem, it's your problem too. If more people think you're pleasant, it makes your life easier, and if they think the opposite... Well... It makes your life just that extra bit harder.
So yeah, I think when people give us the advice to "smile!" it's more for our sake, than it is for the sake of others. I think NTs know about this kind of stuff without even having to think about it, and the idea seems so complicated to us that we would never think they were giving that advice for any reason other than to "tell us what to do", when really they're actually just trying to help with something that's so obvious to them, and we're just getting the wrong idea.