According to a study, New York City's new laws requiring calorie counts on fast food restaurants doesn't appearing to be having any net effect on food purchases. One would think this law would get people to steer clear of fast food, but it's easy to see why, in practice, this doesn't make any bit of difference.
Like smoking, the unhealthiness of fast food is common knowledge. Telling them exactly how many calories are in X, Y, and Z change anything because they're not seen as relevant (ignoring that a high calorie count isn't the same thing as being unhealthy). If you already know a Big Mac is bad for you, knowing it has 576, not 500, calories isn't going to change your decision.
It's too costly to calculate everything down to such fine nuance. Instead, people think in broad categories. Now, if estimated calorie count was off by a factor of 2 or 10, we'd see some real movement. But, on the margin, this information adds virtually nothing. People are good at estimating calories by themselves.