Seems to me that you are desiring a standard for non-citizens that you don't place on the shoulders of citizens. To be honest, I think all Police should have to have a course in civics (many do, as part of their otherwise useless CJ degree).
It would seem to me that the answer would be to make applying for Police work have a national standard of acceptance based on knowledge of laws and their application, with local being able to add to, but not take away from that. I would assume there is some of that. I dunno, having never applied for such a position.
What I do know is that the citizen only route really looks like crap, cus it seems there's a very good sample of idiot cop videos all over the internet. Also, knowledge of something != adherence and application, as demonstrated by not only citizens who are cops, but even a huge amount of lawyers.