This may be another case of Travel Books being full of shite. My bad.
Well, it might be an arcane law on the books, trotted out to hold suspects?
One thing about the Swiss cops, is they loooove to play up the guilt factor. We used to take our boat to the lake on a trailer without something, was it the license plate?? Can’t remember. And sometimes we were pulled over, and it was always, “Well, if you KNEW it was wrong, why did you DO it??” Enough already, just gimme the ticket. :)