The goal of $100 million isn't that Yahoo wants money. They want to be a sexy, ambitious, developer-oriented employer like Google. They want to experiment, and break things, and most importantly identify what is and isn't working. Rather than letting some cut-rate dev produce a middling Japanese Finance page for his whole career, they want a few impressive brands that will draw new, talented developers in. Free food is part of that branding as well.

Buying a promising startup and smothering it with their existing bureaucracy would only hurt their reputation at this point.