Decorator example from Design Pattern Framework

I hope that I don't get moved to the 'hair-splitters' section for that question, but as we're talking here about very basic principles, I just wanted to get everything right :)

In the Design Pattern Framework, there's an example in the solution DoFactory.HeadFirst.Decorator.Starrbuzz. That example contains a base-class, which is called Beverage.

My question is now, why don't we declare this base-class as abstract, as we're never going to have instance of a sole Beverage, but only drinks like e.g. Espresso, DarkRoast, ...?

public class Beverage
{
   public virtual string Description { get; protected set; }
   public virtual double Cost { get; protected set; }
}

public class DarkRoast : Beverage
{
   public DarkRoast()
   {
      Description = "Dark Roast Coffee";
      Cost = 0.99;
   }
}