What if Tweeter and other places that have message length limit ignore the length of the URL in you post?
Then again, people shouldn't click random links.
What does this even mean? The web's primary use case is clicking random links as defined in some context. Shortened links rarely show up in a tweet without any defining context.
And before you say "it's trivial, I could build that in a weekend" I suggest you try it; there is a lot of hidden complexity in something so simple, especially at any kind of scale.
Couldn't they in any case have a convention of putting a hash mark (#) for the URL when texting and then sending URLs in subsequent messages, each message being serially (by time of sending) matched with a # mark?