I really fail to see the "problem". They are different stores serving different economies. They could be making money a lot more in the US store that they can afford to pull the price down as opposed to the UK one. Just cause an item is on sale on amazon.com doesn't mean it "should" be on sale in amazon.de.
I do personally feel there is a significant difference between electronic data and physical goods.
One, if it's multi-language is the same items that's on servers and is distributed via servers with an almost zero duplication fee.
The other has to be manufactured, perhaps in different languages and shipped to different parts of the globe.
Of course, having apps that show you these price differences is not good.
If nothing else, it's bad public relations to do such a thing.
Esp as all others app deals I've seen have been across the board.