This makes a lot of sense for Google. There's really only one form element on Google's home page. That's where the focus should go. There's really no harm in placing the focus there, and as Patrick pointed out, it's a good source of revenue for Google. I believe that is what's termed a win-win.
Granted, this is only a mild irritant, and now that I know Twitter is stealing my focus I can just change my behavior to compensate. The point is that I shouldn't have to. It would be a lot simpler to change the behavior of Twitter's login page than it would be to change the behavior of millions of users.
Another possibility is to check what field currently has focus before you change it. If the user is already using your page, just let it go. If none of your form elements currently has focus, go ahead and suggest a starting point by focusing on the first form element.
Finally, as a compromise between the two solutions above (and this is my recommended solution), just don't steal focus on any page that contains a password field. If there's a possibility of revealing the user's login credentials to passers-by, then just don't take the risk.