Pretty cool device, but apparently there are some serious privacy concerns regarding the saving of information. In Maryland in particular they are showing up all over the place, and causing a debate in the process over whether the information should be stored, and if so for how long.
Personally I think the devices have a very specific use, so I have no issues with them. However, storing the information is another matter entirely. For the use they were designed for, law enforcement, there is absolutely no reason to store the information except in the case of tracking a vehicle that has been listed as stolen, or tagged as possibly involved in a crime. Store it long enough to find the vehicle, pull them over, arrest the suspect. Possibly keep it around long enough for the court case; though I can't see any use for doing so myself since the purpose is to find the individual responsible and at that point you no longer need the information in your system, they already have the number and vehicle on file and proof it was involved in a crime.
I'll say it again: There is absolutely no reason to store and track the information for the millions of cars that get scanned if the vehicle is not involved in a crime. Storing it is just another excuse for the government to track the movements of citizens, an act it has no reason to be involved in in the first place.