You technically can fight that. In tech school (USAF - Biloxi : Radar) the urban legend was about the FCC's ruling stating that any radiated signal within certain parameters you were given the freedom to transmit and intercept. These frequencies covered the bands that cops use on their radar guns, thereby making it LEGAL in all 50 states to use radar detectors and radar jammers.
However, I have never heard of anyone successfully winning in court on a charge of using a detector in a state that outlaws them.
from the above link "Seven states--California, Minnesota, New Jersey, Oklahoma, South Dakota, Vermont and Virginia-and the District of Columbia have statutes barring the use of scanners in committing a crime"
So, if you are going to commit a crime, don't use a police scanner or else??