Interesting.

So, why is (apparently) everyone — including Twitter — sending us through t.co?

Example here[0] where you can see Twitter displays "mlkshk.com/r/K29L.jpeg" in the tweet, downright lies to you when hovering over it with "http://mlkshk.com/r/K29L.jpeg while the real href is http://t.co/fqxnG84t.

So, WWW broken.

[0] https://twitter.com/xkcdrss/status/255158017253838848

I imagine that Twitter sends you through t.co for tracking and analytics purposes. But a lot of Twitter clients just send you to the full URL.

(By the way, I have to check if Twitter forbids bypassing t.co shortened URLs in third party clients).

EDIT: As I supposed, the display requirements say that the clients should send the user through t.co https://dev.twitter.com/terms/display-requirements

And that's why we should be mad at twitter. Sorted.