And Microsoft was rumored to be wanting to buy Doubleclick. So, it's possible that Google thought that the combination of Microsoft+Doubleclick would be more evil than Google+Doubleclick, and extended their "Don't be evil" credo to also mean "Make a potentially very evil situation a little less evil when time and 3 billion dollars permits".

Or, they could be buying Doubleclick just to shut them down... Not very likely, I know.