Even though fake smiles are similar to genuine smiles, they're actually slightly different when you look at the different muscles used to produce the smile. These muscles in turn are controlled by different parts of the brain.
Most everyone can produce a fake smile at will. The brain prompts the zygomaticus major muscles in the cheeks to contract (these are the muscles that pull the corners of the mouth outwards producing a smile).
Apart from trying to decipher whether someone is lying or not, the way I see it, a fake or genuine smile isn't for me to judge. I'll always think it's nice to greet others with a smile. If others have to "fake it till they make it" so be it. I applaud anyone who tries!