Did they work with apple to create a custom version of iOS to integrate with amazon's service, or did amazon jailbreak the devices and installed their own hooks?
The summary makes it sound like they only wipe your app from the device, not reinstall the OS between users.
EC2 is used to run production services with sensitive data.
Most Android/iOS testing is done with made up data, without privileged access to production. So even if it get compromised, it probably not much different from installing an app on jailbreaked device be end user.