Not quite true -- tweets can include in_reply_to metadata, indicating the last tweet responded to, and location metadata, indicating where the tweet was sent. And of course, the account that sent the tweet is metadata, and so is the time it was sent.
And for my money, the reason they wrap every link through t.co is because this way they have analytics on every link clicked on Twitter. That sounds pretty valuable to me.
I know that was the original idea, but I wonder how many people in the last year or two receive tweets via SMS. If I was in charge, it's a tradeoff I'd be willing to make.
The entire world is not the United States. :) "Feature" phones and spotty data service are still very common in certain regions, but SMS works almost anywhere with cell infrastructure (or via sat phones).
And for my money, the reason they wrap every link through t.co is because this way they have analytics on every link clicked on Twitter. That sounds pretty valuable to me.