That's the Amazon Pre-order Guarantee. If they didn't do that, then make a huge stink.
It only applies if the price drops before it ships. Once I bought something and the price dropped after it shipped but before I received it. I contacted Amazon, and they said the price guarantee doesn't apply if it ships before the price drop, however they gave me the credit as a courtesy.
Otherwise I would have returned the first item, and then re-bought it at the lower price, but they saved me that extra hassle.