The cause of health and nutrition is a noble one, but as with many just and good causes, there are some approaches that are more realistic than others. The FDA has kindly regulated the labeling of nutritional information on packaged foods for years. This provides the consumer with valuable information that helps them to make informed decisions about what, and how much, to eat. Perhaps we take this service for granted now, and expect it to be done throughout the food industry. This is not always possible.
Prepackaged foods are prepared to exacting specifications. Calories and fats can be reported because every ingredient has been portioned out to meet the product specifications. Variation between packages must be minimal, or else the product cannot pass quality testing. In short, you can have nutritional labels on prepackaged foods because they have an exact, known content that has been accurately tested, and never changes no matter how many boxes roll down the conveyor belt.
Restaurants are an entirely different matter. Some restaurants, like fast food and pizza venues are highly systematic in their food preparation. Many of the ingredients come themselves prepackaged, and the assemblers (it isn't really fair to call them cooks in this context) are highly trained to use a certain amount of each component. In this case, it is still possible to give a fair estimate of the calories, because the variation is small. Indeed, some restaurants like Subway have been known to do so of their own accord.
Progressing along the food chain, so to speak, we come to actual diners and classier establishments. At this point, there is an actual cook, chef, or team of chefs and cooks preparing the food. They have basic recipes to follow, but variation compounds quickly. Ingredients are fresher, not always prepackaged, allowing for a greater natural variation. Different cooks will use unequal quantities of the various ingredients, and may employ slightly different cooking times or temperatures. The food is not mass-produced, and portions will vary depending on the server. The actual ingredients may even vary, depending on their availability. In short, the likelihood of getting exactly the same meal twice in a row is very slim. This means that the restaurant truly cannot provide you with accurate calorie and fat data. If they do, it is an estimate - a guess - at best.
Requiring nutritional information of a restaurant is a sure way to cripple the restaurant's ability to create excellent meals for its patrons. A chef would never be able to experiment, lest he change the calorie count. (And yes, legislation would have this effect.) Controls would have to be implemented to make sure every dish was uniform, with the end result being that you could just have easily gotten your dinner from a box.
Recognize, now, that dining out is a privilege. Every person could simply make their own meals. That gives them control over exactly what they eat, with all the nutritional data at their fingertips. The choice to indulge - to be lazy and enjoy someone else's cooking - should be just that. Enjoy and indulge for a change - no one meal should have so many calories or so much fat that it will damage a person's health. If someone is eating out so frequently that the unknown calories are a concern, perhaps the better solution would be to change their habits, putting them back in direct control of what they eat. Packing a meal doesn't take long. Leftovers, a PBJ and an apple, a lean microwavable dish - all of these are fair game, and let a person avoid excessive restaurant dining.
As a final parting thought, stop and consider this - if you were in the kitchen, preparing a meal for family of friends, should you be required to provide them with the fat and calorie content of the plate you hand them? Would you? Could you?