It is essential to this story that the order to Apple is not a subpoena: it is issued under the All Writs Act of 1789, which says that federal courts can issue “all writs necessary or appropriate in aid of their respective jurisdictions and agreeable to the usages and principles of law.” Read as a whole, this simply means that judges can tell people to follow the law, but they have to do so in a way that, in itself, respects the law. The Act was written at a time when a lot of the mechanics of the law still had to be worked out. But there are qualifications there: warnings about the writs having to be “appropriate” and “agreeable,” not just to the law but to the law’s “principles.” The government, in its use of the writ now, seems to be treating those caveats as background noise. If it can tell Apple, which has been accused of no wrongdoing, to sit down and write a custom operating system for it, what else could it do?
from The New Yorker - The Dangerous All Writs Act Precedent in the Apple Encryption Case