Imagine a world where SOPA had passed, and everyone who ran a website was legally responsible for everything that their users did.
In that scenario, one way for website operators to protect themselves is to make it impossible to know what their users are doing.
The 3rd party URL shorting service is not storing or associated with the encrypted data and is also not responsible for it.
So this may not be about private data so much as it is about protecting the freedom of information.