And you're suggesting that it does work neatly for Google?
It is profitable for Google. Once you've signed up for an account with adwords, you are more likely to keep it open even once your site is doing well as compared to if you never signed up for an account. This is known as retentive churn (well, it is now anyway). I'm not saying that's why Google does it, but it worked with me. Of course, even with the account open, I'm not currently advertising with them.