Why would Apple change the default search? Well Apple and Google are the main rivals in the mobile market, it makes sense for both Apple to choose an alternative default search engine, as they don't want to strengthen their main rival. I also think Apple's recent 'interest' in robotics (Siri, hiring ex-segway engineer) v.s. Google's robotic cars & Glass, this will be another front that Apple will have to fight Google on.
"1 Billion People Will Use Mobile As Primary Internet Access Point In 2012" http://googlemobileads.blogspot.com.au/2012/02/consumers-lov...
That leaves them with two options, the first being Bing. No way in hell would they choose Microsoft to partner with, with MS' purchase of Skype and close ties with FB you can expect MS to push harder than ever in the Mobile space, over the next few years (assuming they can tie all of their products together neatly).
So that really only leaves Yahoo for search, which works out well for both companies; Yahoo get to survive the battle of the big 3 and still appear somewhat relevant and Apple are no longer strengthening their main rival. I really have no idea what I'm talking about but it sounds interesting.
tl;dr - purchase of summly was marketing and survival by Yahoo