Does AWS have to pay a license fee to maintain a pool of Windows instances? Who foots the cost of the license if a prepared/pooled instance is never allocated to a customer?
There has to be a custom deal in place, but it could be similar in spirit to a hot standby MS SQL Server instance where you only pay for one license but have a mirror of the machine ready to go if the first one goes down (my knowledge of licensing was only tangential to my dev work and could be several years out of date now).
There are MS license agreements for cloud solutions providers that allow license usage up to a certain limit, with monthly reporting of actual usage for billing purposes.