People are missing the point of this deal. Groupon is a scalable way to market Google ads to local business.
Google already sees how much Groupon spends on Google ads.
From that they realized that they can take that expense out of the equation, and Groupon becomes even more profitable.
The opportunity cost for Google of serving some junk ad vs a targeted Groupon is low. There is some risk involved, but there is a huge potential for reward.