Having been a brother of the badge and an instructor for quite a while, I must reluctantly admit that cops......are people too. And although MOST departments work very hard to screen out candidates who are...abnormal...there are always a few who make it through the process or develop bad habbits at some point along their career.
Unfortunately, that sometimes means that an officer might be enough of an egotist that they allow their personal preferences and values to seep into their duties. Some might say something is illegal, when it is not a formal statute or ordinance, just to flex their perceived authority. I've even seen a couple of officers over the years who have gone so far as to issue a citation, and go to court, and be informed by the defense counsel that there was no such law.
Three possibilities exist in such a scenario: 1) Officer just made s__t up, and didnt think the citizen would challenge the citation. 2) Officer misinterpreted "his understanding" of what could be a poorly or vaguely written statute which allows for "officer discretion" (I hate those). 3) Officer just didnt know.
On the flip side....there is, of course, always the potential for completely assinine laws to exist on the books, and while a good officer might shake their head and choose not to enforce legislative idiocy...another officer might be having a bad day or need to flex his ego or be bored or for whatever reason he might decided that you win the prize and get to be the guy who gets legitimately cited for an obscure offense.
Some folks wave those off as "technicalities" but you'll find that the term "technicality" and "Rule or law"are interchangable dependant upon which side of the courtroom you are sitting in.
In short...find, print, and read and know your states motor vehicle laws. If you encounter an officer who you think is citing you incorrectly (you are people too and could be wrong just as easily as he) just go with it, dont argue or get all bowed-up. The only thing more dangerous than a bored cop is a pissed-off police officer. Take the citation, followup with the patrol supervisor the following day, and with the court if need-be.
the moral to all of this.... not every "law" quoted by cops...is real.
In fairness to the profession, a vaaaast majority of officers truely do try to do the right thing and not abuse the powers that are afforded to them by the citizens of their community.
Sadly, there are still some of these out there and even ONE is too many:
Cartman Respect My Authoritah - South Park - YouTube