While cream soda tastes great, especially when mixed with sugar, cream, and vanilla on top, it's just not the same. The Butterbeer sold in the park is non-dairy, as has been pointed out earlier in this thread.
That rules out butter and cream. Cream soda certainly can be the base, as there's not actually cream in it, but I believe it's more than just ordinary cream soda in there. And then there's the foam. How do you create a thick, frothy vanilla foam without using a single dairy product? Cool Whip is considered "non-dairy" though it does contain casein, which comes from milk. So if something like Cool Whip is used to create the Butterbeer topping, would Universal still be able to call that non-dairy?