It has to do with Disney's attempts to create more balanced meals for children. In order to hit all of the food groups they need the appetizer included. At least that's what I've heard.
Another thing is that many of the kids meals are pre-plated and the pre-plated meal includes an appetizer. In cases where a meal is 'pre-plated' for an adult (Cinderella's Castle, LTT, WCC Skillet, Fantasmic Dining Packages etc) the adults also receive an appetizer.