Maud - I know it's hard to see at first but if you scroll down the page a bit, you'll see this:
Product Details * Downloading: Currently, this item is available only to customers located in the United States and who have a U.S. billing address.
Also, for those who are in the U.S. and want to download this game, Amazon is offering a FREE game download - here are the conditions:
Buy a Game Download, Get Another Game Free
For a limited time, purchase any game download* from the Game Downloads Store and get a second title from the qualifying list below for free. To qualify, purchase your game download and within two business days you'll receive an e-mail letting you know that a credit, good toward an additional game download, has been applied to your Amazon.com account. To use this credit, simply follow the link in the e-mail to return to this page and select your free game. The price of the qualifying free game will be deducted from your order at checkout.
Offer valid until 11:59 p.m. Pacific time October 1, 2009. Free game must be redeemed by 11:59 p.m. Pacific time November 1, 2009. Limited to one free game per customer.
It's tricky because you won't see this offer every time you visit the "Granny" game page, so you might need to refresh the page until it comes up.
Hope this is helpful!