Why do people say if you get off the atkins diet, you gain the $hit back no matter what?

my friend did it to lost 15 pounds and it worked, but she was smart about it when coming off. she slowly introduced carbs back into her life, but healthy carbs instead of bad ones such as white bread, pasta, white rice, so on. she took it slow with whole grain foods and wheat foods. she didn't gain anything back really. my brother did it, lost weight but gained a lot of it back because he was eating things like brownies, cake, fried chicken, pizza, tacobell, and so on. so why do people say you gain it back no matter what?