CROSS APPLY Vs OUTER APPLY Example Messed Up?

Hi... I'm reading the MS Press 70-442 Self-Paced Training kit, and I'm having problems with this example.
I'd like help getting it to work correctly, or understand why it is isn't working the way I planned.

On page 67, the lab is about the APPLY operator (CROSS APPLY and OUTER APPLY). I first have to input a sample table-valued function into the AdventureWorks database:




Code Block
CREATE FUNCTION fnGetAvgCost(@ProdID int)
RETURNS @RetTable TABLE (AvgCost money)
AS
BEGIN
WITH Product(stdcost)
AS
(
SELECT avg(standardcost) as AvgCost
FROM Production.ProductCostHistory
WHERE ProductID = @ProdID
)
INSERT INTO @RetTable
SELECT * FROM Product
RETURN
END



and then run a sample T-SQL statement





Code Block
SELECT p.Name, p.ProductNumber,
Convert(varchar, cost.AvgCost,1) AS 'Average Cost'
FROM Production.Product p
CROSS APPLY fnGetAvgCost(p.ProductID) AS cost
WHERE cost.AvgCost IS NOT NULL
ORDER BY cost.AvgCost desc

My problem is with the WHERE clause... According to page 56, CROSS APPLY returns only rows from the outer table that produces a result set, so why do I need to explicitly filter NULL values?

When I remove the WHERE clause, the query retrieves lots of NULL AvgCost values.

Again, according to page 56, it is the OUTER APPLY that returns all rows that return a result set and will include NULL values in the columns that are returned from the table-valued function.

So, in short, I don't see the difference between CROSS APPLY and OUTER APPLY, using this example, when I remove the WHERE clause?

(Please refrain from introducing another example into this question.)