The reason being that people aren't buying PCs any more as the ones they have are perfectly adequate (or they're using phones/tablets instead) , which means no new Windows sales...... so they want you to rent it in the future. No company ever gave anything away free without expecting payback later, and Microsoft is no exception. They've already started a Surface rental scheme in the US and monthly subscription for W10 (or at least the bits of it that make any useful things work, the basic free version will be hobbled to the point of useless) won't be far away, along with Office 365.