That's weird. When I try, it says "I think you're trying to buy from Amazon.com" and redirects me away from the German site.
edit - Yeah I just tried and I get this message:
Wir konnten Ihren Einkauf nicht abschließen. Ihr Kindle-Konto ist auf Amazon.com angemeldet. Kindle-Titel, die für Ihr Land verfügbar sind, erhalten Sie auf Amazon.com.
Basically saying "your account is linked to Amazon.com, to see titles available in your country go to Amazon.com".
I actually signed up for a brand new account on Amazon.de, and when I go to the Kindle version of a book I want, instead of a purchase button, I get this message, which I understand the translation but I'm not sure what they're trying to communicate to me.
Kindle-Titel sind für Kunden in DE unter Amazon.de verfügbar.
Setzen Sie Ihren Einkauf im Kindle-Shop von Amazon.de fort.
Clicking the "im Kindle-Shop von Amazon.de fort" link redirects me to the same page with the same message.
Kindle; so e-books right? That's digital content, and that suffers from the same problem (and silly restrictions) Netflix is having. Real — sorry — physical books might work.