Pseudo-legal language is just one more example of popular misuse of technical jargon. I strongly suspect that the phrase "grand theft auto" is not used in actual US legal jargon. My suspicion is based on several examples of misused technical jargon that I know for certain to be wrong.
Example 1: tarmac. Whenever aviation makes the news, the media always says tarmac when referring to paved areas of an airport other than runways and taxiways. As a licenced pilot for more than 50 years, I can promise you that NOBODY in aviation says tarmac. The paved areas of airports where aircraft are parked is always called "the ramp."
Example 2: heart failure. Whenever someone dies of sudden cardiac arrest, the media always says heart failure. In fact, heart failure has nothing to do with sudden cardiac arrest. It is a particularly miserable way to die in which you slowly drown in your own bodily secretions.
Raymott, if you are out there, back me up.