I'm with ya, I have the feeling the especially women do that, and they seem the most confused is you don't reply by smiling back. As for guys, most of them look kinda stupid whe they smile (I know I do), and I've known some "sunny boys" back in school that where so full of themselves and their "charisma", they had no other facial expression that an inbred smile.
Going back on track, I simply try to ignore it, because smiles can mean so many different things, but with inredibly subtle differences, that it would take me an eternity to interpret them correctly if I payed attention to every single one. Sometimes I try to smile back at people I know or when I wanted to show them that I am in a good mood, but otherwise, I'll stick to the good old poker face and just go on. For me, smiling back - especially at women, increases the chance of approaching me and starting some BS small talk conversation with me, just giving me another opportunity to make a fool out of myself because they caught me unprepared.