What is null key encryption?

In the movie Dark Knight, the Batman builds some ultra powerful sonar monitoring system and encrypts it with "Null Key Encryption".

I was reading RFC 2410 but couldn't comprehend it. It says something like this:

However there are cases when only authentication and integrity protection is required, and confidentiality is not needed or not permitted.

And in the end, the Batman says to another character:

Type in your name when you are finished.

If it's that simple, why encrypt it?