"But if you received Windows 10"
So those who got a free upgrade to Windows 10 should get the ads to cover the costs?
Anyway, you're utterly wrong if you believe that Google slings ads to pay for development costs - it earns from ads far, far more than development costs - just look at Google's profits, do you believe they are those of a "small developer"?
Google understood that free apps - preferably web ones so it can use open source code without open sourcing its own one, and with only basic functionalities - are the best way to lure users into its data slurping ecosystem, and that's what Microsoft is trying to imitate. You become the product which advertisers pay for...