Dispite having briefly lived in Utah, I don't know if it requires cars to have a front license plate. I know many states only require a back license plate and therefore a police officer can't pull you over or ticket someone for have no front license plate. Utah could be strange and require a front license plate but you should check out the DMV regulation for yourself. I wouldn't trust what the DMV worker said.
My gut feeling is that the police officer is pulling you over because he has watched too many movies and thinks that if you are driving a nice, fast car at night you must be up to no good. Police officers are funny that way. It is also the reason why many drug dealers and other criminal can't be convicted because police officers are unable to explain what their probable cause was for the stop in a manner that a decent defense attorney and judge won't openly mock in court. I know DAs try to teach police officer that their gut is not probable cause but they can only do so much.
