But the end user probably trusts the machine 99%, so what if you load it with something malicious, send it back to Amazon and wait for it to be sent to another customer and thus hack their network? That is if Amazon does not completely wipe their drives (Hide it somehow?).
Obviously this device has been designed to be a multi-time use device.
Amazon definitely do NOT have physical control over this box. They would need to do a complete low level reflash of every single bit of firmware on there, every time it came back. That's not actually that inconceivable in a enterprise grade server, and I hope to see some interesting details about that.
But still... just imagine some of the fun HDD firmware hacks making their way onto this. Or NIC firmware. Or even just the embedded Kindle being rooted, and used to sniff out Wifi networks, report its location via 3G, etc.
Not to mention the obvious data recovery attacks if the disks have not been wiped to the highest levels.
I agree with parent comments, and assume Amazon would put this on its own untrusted VLAN when it comes back. But would they weight it first to see if any pwnies have been inserted into the box? Visually inspect inside to see if physical components have been removed to ensure the weight is actually the same, despite a pwnie inside?
I really hope Amazon put out advice to their customers on how to connect this - ideally it should just be on a point to point link to a sacrificial server containing the data.
But yes, if you assume this device is treated as malicious at both ends - just like an unfiltered internet connection, but 10x worse - and that the client software that is doing the load/verification, or unload/verification is doing decent input validation, and your assumption that the user is doing their own encryption prior to transfering it to the device, I agree.