Most users aren't aware that Yahoo is powered by Bing, so the fact that Bing overtook Yahoo in search shows that Bing has gained (or Yahoo has lost) significant ground. This isn't searches powered by Bing, but searches actually on Bing.com.
Also, Microsoft doesn't really want people searching through Yahoo.com, in a few years time they could power their results through Google and Microsoft wouldn't be better off. They want brand recognition for Bing, so gaining market share for Bing itself is better for them.