The normal Microsoft service provider license agreement (SPLA) forces you to buy subscriber access licenses (SALs) on a monthly basis. Amazon may have a custom

The normal Microsoft service provider license agreement (SPLA) forces you to buy subscriber access licenses (SALs) on a monthly basis. Amazon may have a custom deal with MS for hourly Windows pricing on EC2 that wasn't extended to Office. It also may have been a marketing move, since enterprise IT folks actually prefer the predictability of simple monthly pricing over utility-style hourly rates.