The ads you see are generally the result of an auction. When you use Contributor it bids in that auction for you, and typically wins it for a small fraction of a cent.

Publishers generally don't have to opt in: if you're a publisher advertising with AdSense, and a visitor shows up who uses Contributor, they'll see "thank you for being a Contributor!" and you get a micropayment from them into your AdSense account.

(Disclaimer: I work for Google, on things completely unrelated to ads.)