If ignorance of the law is not an excuse for a civilian, then, a law officer should be expected to have just a much knowledge of the law as the civilian. In fact, the law officer is expected to have a lot more knowledge of the laws which he/she is enforcing.
Sadly, they will never buy your reasonable argument.
That reasonable belief was dispelled only by a ‘surprising’ appellate court ruling that for the first time construed a traffic law on the books for more than fifty years to require only one functioning brake light.
Apparently, there was a new interpretation of the law based on this case. There’s no way the officer could have known.