Its deeper than the obvious
first of all they would be financing another party to buy yahoo
second there non search things google might want from yahoo even if those things seem small, if you got tons of money and your extremely confident you will get more than your cash back, why not set up a situation where you pluck cherrys off of the yahoo tree.
And its world wide components that dont fall in US or EU "anti trust"
Yahoo's hold the largest Alibaba share block!
In 1997 Microsoft invested in Apple just to keep a competitor around.
So all this has to develop, Google smart enough not to endanger there getting $20 bucks because someone laying on the couch clicked a certain click lol.