You are correct that law enforcement does not provide the authoritative interpretation of the Constitution, but you seem to imply that they should operate under ignorance of the Constitution. This is wrong.
Law enforcement must have an understanding of the Constitution, and they must uphold it. For example, it's a dereliction of duty to always argue that any search is reasonable.
Law enforcement must have an understanding of all laws (for their jurisdiction), including the Constitution. However, when there is a legitimate question, it's not up to law enforcement to make the final judgement calls. It's for the judiciary to make that call.
It's fair to say that this is an argument that has been building for a while.
As such, it's very appropriate for the courts to make this call. And once that decision is made, it's up to law enforcement to act accordingly.
Note: I think the California court will reach the same decision as the Brooklyn court. But I don't have an problem with the DoJ/FBI raising the issue.
That's not actually true. The executive and the legislature all have to swear to uphold/defend the constitution and it takes all three branches of government to violate it. If Congress says that something is unconstitutional and refuses to pass a law permitting it then the executive can't do it. (The FBI is nowhere in the constitution, its very existence is at the will of the legislature.) If the executive branch says that something is unconstitutional then they can refuse to do it. Nobody can force the executive to prosecute someone under a law they think is unconstitutional.
People only see the courts as the arbiters of the constitution because they're last. You only get there if the legislature is willing to pass the law and the executive is willing to enforce it. But that is a piss poor excuse for the other branches of government to neglect their oaths.