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.
You don't have to make a huge stink, I contacted them via chat, and they took care of the problem with no issues.
I've had the same experience with Amazon, where the price of something went down not long after purchase. They pulled the same thing on me - first it's sorry, nothing we can do, we won't refund the difference; but if you don't accept that and press a little more, you'll get a "credit" for the amount in dispute toward another purchase.
Must be on their script. First goal is to keep your money; second is to keep your business. It's rather obvious.