Since tags are publicly displayed, a cop could choose any tag he wishes (such as who he is following at the time) and run it. There's no law saying he can't. As to whether he denied that to the couple, the courts are on record as allowing cops to lie to the public. Whether that ought to be the case is another matter of debate, but currently it's blessed.
While you may think so, the Greer police, who are investigating the incident clearly understand that Officer Hellfire's motivation for running the tag is relevant, since they are investigating the reason. If you don't get it, just think: Suppose he was mostly running the tags for blacks, with no other justification.
The difference is, we all understand from decades of "education" that blacks are protected, but you assume that Druids deserve no protection against being targeted for special enforcement actions.