content: Because Windows 10 is a monetization platform, via data mining, advertising, subscription selling, etc.  It's not a traditional desktop o  38d5  perating system anymore.  Windows 7 and 8.1 are not giving Microsoft these same opportunities, so as far as they are concerned, whatever was spent to buy the OS is past revenue, and Windows 10 represents future revenue, not from the OS itself, but because of the platform it provides to future monetization through using it.  It's free the same way that Amazon provides a "free" shopping portal, Google provides "free" search, Facebook provides "free" social media.