I'm unaware of Valve using Steam to delete user saved games or purchases
or of any reassurances from Amazon that the titles will be unlocked if amazon or the ebook service were to go under.

This is possibly the most important part. This is what I was talking about with DRM that restricts what the user can do. Steam has no restrictions on how many times you can download, or on how many machines you can download to.

Quote Originally Posted by Amazon FAQ
Most books and other non-subscription items you purchase from the Kindle store may be simultaneously accessed for your personal use on up to six Kindles (or Kindle compatible devices, such as Kindle for iPhone) registered to your Amazon.com account. If the limit is less than six kindles for a specific title, you'll see the message "Simultaneous Device usage: Up to X simultaneous devices, per publisher limits" on the detail page.

If you reach the device limit and wish to replace one of your current devices with a new one, you must first deregister and delete the content from the device you wish to replace before you can access the content in question from your new device. For device registration instructions, please see the Help page for the device you wish to register/deregister on the Kindle Support pages. There is no limit on the number of times a title can be downloaded to a registered device.

Subscription content can only be downloaded to one Kindle at a time and cannot be accessed from Kindle for iPhone.