Man if that could work I'd be so happy. People just say they didn't know better and in the US at least you often need intent, so this would be very difficult to work in current US law. Hopefully it will work better in Wales
The number of people in jail/prison in the US because they didn't’ know better, is staggering. So the standard is already in place. I see no issues with dumping the worst crooks out there, in prison for being ignorant.
Generally a cop's remit doesn't include extensive knowledge of law - that's why we have lawyers and judges.
Expecting cops to have that kind of knowledge isn't useful, and contributes to current problems.
Even the most well-intentioned, best-educated cop can make mistakes (as we all can). So simply never argue with them - let them arrest you without complaint (because if they're going to arrest you, there's nothing you can do that won't exacerbate the situation), any such issues can only be resolved in court - that's the place for arguments.