No mention of how they are dealing with the Windows licenses. A quote from the Microsoft VDI and VDA FAQ
"Currently, there is no SPLA model for Windows VDA. Hence, customers who subscribe to desktops from a third-party hoster will need to pay Microsoft for a Windows VDA license for each device accessing Windows client virtual machines in the datacenter. Additionally, hosters need to ensure they isolate the hardware and other resources for each company. Any hardware running an instance of Microsoft software (OS or application) must be dedicated to a single customer. For example, a SAN device that is not running any Microsoft software may be shared by more than one customer; whereas, a server or SAN device that runs Microsoft software may only be used by one customer."
So unless Amazon have some kind of special deal with Microsoft, there is no way that this can work running Windows 7.