Indeed, even if Microsoft believed that it would be prevented from buying Yahoo! on antitrust grounds, it might make sense to push for the deal, if only to force the antitrust authorities to take a serious look at issues of market power in the online search and advertising market, which would inevitably lead them to Google.
By responding on February 4th to Microsoft's bid by offering whatever support Yahoo! might require, Google may have walked into Microsoft's sucker punch. For whatever adverse effects on competition might result from Microsoft buying Yahoo!, the impact of a closer relationship between Google and Yahoo! would surely be worse.
An interesting viewpoint into Microsoft's approaches towards Yahoo. Could it all be a set up?