Attorney Bob Rogers may have been Mayberry's intern deputy until just moments before ("Barney's Replacement," Episode #33), but his car is no longer an official police vehicle and therefore is fair game for Fife the Fierce.
At least Bob didn't violate Rule 8, Section B (Ordinance 115) - parking in front of a fire hydrant. That would have been real trouble. By the way, Bob's license plate number is DC-269.