Well after a prompt reply from Amazon in regards to their Appstore, I am not entirely certain what to think. It really feels that they are putting a majority of the blame directly onto the developers.
Updates for your apps will automatically be made available in "My Apps" once the developer submits the app, and it's approved by our Review Team. Depending on when the developer submits the app for review this may result in the update being available after other marketplaces.
Our goal is for Amazon Appstore customers to have a great experience with every app they order. As a result, we test all apps and updates prior to making them available in our store to verify that each app works as outlined in the product description, does not impair the functionality of the mobile device, and does not put customer data at risk once installed.
You can let the developer know you'd like to see their updates made available in the Amazon Appstore by submitting feedback on their website.
Personally this sounds a lot like Amazon is passing the buck onto the developers for taking forever to maintain updates on their Appstore. This is not beneficial to Amazon, when the Android Market is able to keep up to date and have new updates churning out almost regularly by the same developers.
If this is truly an issue that lies in the developers hands, then are they shooting themselves in the foot by not maintaining regular updates to their products on the Amazon Appstore?