one of my best mates is a cop and told me that when off duty they ARE aloud to book you and infact are encouraged too, and technically arent ever off duty unless they've been drinking.
Just because they are cops doesnt mean that they cant own a Skyline, 200sx, WRX, EVO etc..
Most wouldnt bother booking you coz it means a lil more paper work, but im sure if you were caught when theyre having a bad day or they were just in a cock head mood..........