At first they are going to want you on a more balanced diet, try to remember they are working with you to promote healthy eating and lifestyle.
Once you have gone through the program and if you are accepted into surgery they will go over what you need to focus on after surgery, but for now they probably just want you to balance everything out and eat in moderation.
Remember anything you eat now, you can eat after surgery, you will just be able to eat much less.
They want to guide you to a better understanding of nutrition and a balanced diet to maximize your chances of making better choices in the future.