And they do it on the onClick event, so you are mislead to believe that the link is clean (as show on your status bar on link mouse over, until the very last second)
And they do that on GMail, calendar, and even on mobile apps such as google voice and hangouts. (i.e. if you long press and copy, it is a clean url, if you touch the link, it opens a tracking redirect on the browser --which chrome hides. But you should be using firefox on android as well)
Firefox (desktop and mobile) and Chorme (only desktop because google don't want you installing adblockers on their browsers) have an extension called "Skip Redirect" that works-around this problem.