Official Google Blog: I clicked to buy and I liked it
By continuing to build on the two giants in online retailing, and not splitting the market, Google (in my opinion) did the right thing. Not that Google could not get away with their own Google store, but it would be a distraction for a company that has traditionally wanted visitors to get out of their own website and get going on the web.
This co-operative move further strengthens Google's position in the industry as a portal, something that is to serve as a gateway to the web and not replace other components of the web.
Wish you all the best, Google!