Steady on there, a good chunk of the legal system exists mainly to interpret and argue about what the law is or when certain laws should be applied. Even a panel of High Court judges can disagree, whereupon a verdict goes by a majority vote, which is absurd if you think about it.
If the law was so obvious, lawyers would not be necessary. You can not reasonably expect a policeman in the street to know much about the law.
Every now and then, a court will find a cyclist guilty of "obstruction" for riding in primary. To date, these have always been cleared on appeal to a higher court, nonetheless it proves that if a court can get the law wrong, then it is entirely unrealistic to expect the police to always get it right.
"then it is entirely unrealistic to expect the police to always get it right."
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