They basically want to get you to buy their product. We have all heard of or even know people, that we are sure can eat anything and everything and not gain weight. They happen to be genetically blessed and unfortunately for us, make up a very small percentage of the population. The fact is that most people do have to watch what they eat or if they don't, they exercise to burn off the calories. Diets that tell you that you will lose weight without having to exercise are stopping you from developing healthier habits.
Exercise is very important to long term well being.
When making lifestyle changes, one of the keys to long-term success is a positive, constructive support network. Many diets sound great but have no scientific backing. Take a moment to find some literature that will support the claims of the diet program or author of the book. Even if there is data, check to see how recently the research was done. If the data is old and there is nothing new to replace it, move on to something else.