No offence Thermo, but Police Officers have a very basic and limited understanding of the law and in many cases different officers will interpret things differently. An officer can not be relied upon to know specifics or all aspects of every part of the law which he polices
That is why people are arrested "on suspicion" of an offence.
The beat bobby or traffic cop will have a limited knowledge of some common situations pertinent to their beat or role, or prevalent in the area they police.
But it is not their job to know all of the law or interpret it, or choose which area of law to prosecute under. The role of the police is to investigate possible offences and apprehend offenders, and not to enforce the law.
There are many, many people who know more about law who don't actually work in the Police.