The real question here is: why don't police refuse to arrest people under a statue that does not jive with the constitution?
Because if they don't make the arrest they can get into trouble. Especially these days when everything is video and audio recorded. They can lose their jobs, lose their pension and basically have their lives destroyed.
Some police (in NJ) are also anti-gun. Not all of them and I don't think most of them are. But there are some.
Secondly how does a jury convict a person who gets arrested and charged under a statue that does not jive under with the constitution?
Sometimes the jury is often not given all of the facts. In the case of Brian Aitken, the judge gave the jury instructions which deliberately omitted the exception to possess firearms when moving from one residence to another Aitken and his attorney had claimed that he was moving and thus should be allowed to possess. Absent that, it was a slam dunk - Aitken had a firearm and he was illegally in possession because he did not have a permit to carry.