1) When MS signed a non-exclusive agreement with Twitter, they must have been 99% sure that Google would be in on the Twitter action as well (i.e. I doubt they ever thought they were one up on Google), and
2) The point is that Google is not already there. All they have to show for themselves is an agreement so far. (MS has http://www.bing.com/twitter up and running already.)
Being first to market isn't everything, but I still think this is a noteworthy exception to the norm of recent years.