Zagat has recently made moves to expand their ratings from food / travel into new verticals - eg rating doctors.
Have to assume that Google foresees expanding the Zagat ratings quickly to all things local.
The Zagat "brand" resonates huge as a trusted consumer brand and having that rating attached to listings (exclusively) makes Google's search results more meaningful than competitors (the perception anyways).
Smart.
Reference - http://www.prnewswire.com/news-releases/wellpoint-taps-zagat...