It's because if they claim to have the eating disorder that is not eating enough then people won't call them out for eating too much. Are you going to tell the person who almost starved themself to death in an effort to be thin that they are now fat and are eating too much? No, because they might go back to their old way of killing themself. Eating disorders, rightfully, will get you sympathy, but the fats ruin the visibility of the issues for their own selfish whims.