I had heard at one point that some people smile during the mugshots so that, if those pictures get out into the open (like those pictures did), that they don't really look like booking photos and can't really be used to humiliate the people in them effectively. That applies more to celebs and politicians but I guess it works just as well for everyday people, if they are worried about photos like that getting out. It looks more innocent than having that usual glum stare that most people would have.
Tom DeLay (former House Majority Leader) smiled during his booking photo and nobody really circulated that photo to discredit him (though, of course, getting booked to begin with did the damage). Mel Gibson tried to smile during his but he was obviously still very sauced at the time so it didn't come off as well.