Why would Apple (or anyone else) want to be in the loop?
Why do they want to be in the business of determining which law enforcement requests are legit? That's not exactly an easy problem, either. You could probably assume an organization like the FBI could have a pretty solid process for who gets to make such requests, under what circumstances, how they are passed to Apple so Apple knows they are legit, etc.
How about a 10 man police department? Do they get to make requests themselves, or do they have to forward them up to a larger organization? If they have to request the help of their state police, or the FBI, then those organizations have to create a way to determine whether it is a legit request or someone is going outside the law hoping to steal his ex-wife's phone and illegally spy on her.
That doesn't even get into other countries, who might not observe the legalities that would (hopefully) be observed in the US. If Apple decides a country is using their police powers to spy on their citizens by unlocking their phones when they are not guilty of any crime, do they get to say "sorry, we won't do unlock requests for country X" and then country X may ban sales of their products in response. Or if they decide misuse of unlocking is not their problem, and respond to all such requests, then they have to deal with the blowback from people back home who will accuse them of helping a country violate their citizens human rights and maybe have congress looking to sanction them for helping evil country X. It is a lose/lose for Apple either way.