NYC is already complaining that they have a lot of phones that they want to have opened. Also, what about the terms of service? Any entity - municipality - that tampers with the software is in violation and should be sued.
Hmmmmm. You don't suppose that NYC authorities would run screaming to court to get those phones unlocked if they caught wind of Apple having a tool to do so, do you?
Them, and every other state and local law enforcement agency. And that's before private lawyers get in line, too - you think a divorce lawyer might want to see what's on the other spouse's locked phone?