Because they are obligated to maximize profit on everything they sell, so including the bump case would bring the iPhone 4 profit margin down by a small amount overall, while selling the bump case as an individual optional product as they are doing could be giving them 14,000% interest or more, who knows.
Can't be more than $.15 to $.20 to make one in the mass quantities they're having made, so it's insanely great profit on it which looks damned good on a quarterly report breakdown. 🙂