I was a "professional" photographer for a few years. I worked for both Walmart and Sears Portrait studios, respectfully. We often took passport photos and we were instructed to tell the clients not to smile. When I asked why, I was told that there are a couple reasons.
First, a passport photo is a serious thing-it is your proof of identity on officially recognized paper, accepted throughout the world. Second, the main reason that you can't smile (or wear a hat, etc.) was because the photo is supposed to look like you for identity purposes; smiles and coverings distort the face. Maybe they are secretly using facial recognition software...who knows?