Except they pay $500 million cash upfront just to submit the order with the vendor being fully aware that Apple is under no obligation to actually purchase the full order.Call a pizza place and order 30 pizzas.
Go pick them up but say you actually only want 5.
Now the pizza maker is stuck with 25 pizzas, you know they will have to get rid of it now so you offer them a buck each. Something being better than nothing you accept. Repeat next week.
If the vendor is truly getting screwed in the deal next time they will change the terms of the agreement.