The Amazon Appstore is a walled garden like it's sister App Store on iOS (I wonder if I pissed Steve Jobs off just then). So, when building the Appstore, I wonder if Amazon just downloaded a bunch of apps from the Android Market and reposted them in their own app store. And from then on out, they would take apps by submission only. That may be why they have a really old version of it.
Come to think about it, the apps on the Amazon Appstore don't seem to be updated all that often. It makes me think that they did just copy a bunch of apps without the devs knowing, so the devs don't bother to update them...just the Android Market ones.