Decorator Pattern from Head First Book wrongfully used?

Currently I am reading "Head First Design Patterns". As you may look on pages 24 and 25 of the PDF here, I have doubt in given example.

Why don't we make something like:

public abstract class Beverage {

String description = "Unknown Beverage";
Double cost;
ArrayList<Topping> toppings;  // allows duplicates


public void addTopping(Topping topping){
toppings.add(topping);
cost+=topping.getCost();
}
getter/setter of description
getter/setter of cost
}



Public class Topping{
 String description;
 double cost;

    getter/setter of description
    getter/setter of cost

}

Then answering questions on page 25:

• Price changes for condiments will force us to alter existing code

  no they won't we can manipulate cost of the topping by setter.

• New condiments will force us to add new methods and alter the cost method in the superclass

  no, the method is the same.

• What if a customer wants a double mocha?

  not a problem

Here we can also add Builder pattern.

Why should you use the decorator pattern in this scenario? Is my solution not enough?