Your point illustrates a common case with cops: they simply don't know many of the laws they try to enforce. My local cycling club (in neighboring Rockland County, NY) had a patrol chief for the Rockland Sheriff's Department speak at a meeting and he was initially incorrect on fairly simple things like whether NYS law requires you to carry ID to cycle (it doesn't, though most find it practical) and on and on.

Sure, these guys are humans and humans make mistakes. But it gets "in my craw" that they're paid more than 100K a year to enforce laws they don't know, when knowing the law is the most crucial part to effective enforcement.