"There are all sorts of interesting (and frustrating and challenging) legal questions raised by the FBI's use of the All Writs Act to try to force Apple to build a system to allow the FBI to hack Apple's customers. But there's one interesting one raised by Albert Gidari that may cut through a lot of the "bigger" questions (especially the Constitutional ones that everyone leaps to) and just makes a pretty simple point: the DOJ is simply wrong that the All Writs Act applies here, rather than the existing wiretapping statute, the Communications Assistance for Law Enforcement Act, or 47 USC 1002, better known by basically everyone as CALEA. CALEA is the law that some (including the DOJ) have wanted "updated" in ways that might force internet companies and mobile phone companies to make their devices more wiretap-ready. But that hasn't happened.
And, as Gidari points out, it seems clear that CALEA preempts the All Writs Act and explicitly forbids what the FBI is requesting here. The DOJ is claiming that CALEA doesn't apply to Apple:
Put simply, CALEA is entirely inapplicable to the present dispute [because] Apple is not acting as a telecommunications carrier, and the Order concerns access to stored data rather than real time interceptions and call-identifying information"