I’m not very happy about this, but I guess it’s about monetization.
Actually I see it more about security (and by extension, maintaining Twitter’s reputation), as this gives Twitter a way to globally disable links that go to Bad Places. However the larger issue of interest is URL shorteners in general. If your service/product can reasonably be thought of as a feature of another service/product, your business is in trouble from the get go.
Here is the original notice.