Just as you would say "I'm going to have a cup of coffee" instead of "I'm going to have coffee in a cup." In order to enjoy your coffee you MUST have the cup first -- not just coffee in thin air and put a cup around it...

So, since the Reeses cup is made of chocolate and the filling is peanutbutter (the cup came first and was filled with the peanutbutter and 'chocolate' gets top billing), and it's defind as chocolate and peanutbutter...