You've got two result-sets there - SSRS will only read the first result-set and ignore the second. UNION ALL them to get the expected result, like below. Note the NULL ProductDate, which makes both result-sets have the same number of columns, required by the UNION operator.

SELECT ProductID,
  ProductName,
    NULL AS ProductDate
FROM Product
WHERE @OutputType='WEB'
UNION ALL
SELECT ProductID,
  ProductName,
  ProductDate
FROM Product
WHERE @OutputType='CSV'