AN EXPLANATION OF THE LEGAL RATIONALE (SUCH AS IT IS) BEHIND A FEDERAL COURT ORDERING APPLE TO HACK INTO A TERRORIST’S IPHONE

When I first heard about a federal magistrate’s order, made at the request of the FBI, that Apple must provide to (actually, create for) the government the means to unlock a terrorist’s iPhone, I immediately thought like a lawyer.  How can a court order someone, who isn’t even a party to a prosecution or investigation, to act to assist law enforcement in obtaining evidence?  That’s not the same a court issuing a subpoena to someone who’s not a party to the case for records or data that’s relevant to the case.  No, the court is telling Apple that it must make something to create access to the data, i.e. create software that will allow the FBI to get data from the terrorist’s phone.
After some looking for an explanation of the legal rationale for the court’s order, I finally found one today.  There’s a federal law adopted in 1789 (you read that right, and not it’s not a typo, a law adopted 227 years ago) that “empower[s] courts to issue orders that are necessary to carry out other legal functions,” according to the item at the link.  The FBI relied on that law to get the court to order Apple to have its engineers create new code to unlock the phone.  
That seems like a pretty weak rationale to me.  Does that law really allow a judge to order a company to make something so that the government can get access to data on a product the company sold?  
I’m also wondering about the procedure that was employed to produce this order.  According to something I read somewhere else, Apple wasn’t informed of the proceeding that led to the order.  I’m guessing (I used to be up on this stuff, but my employment as a law clerk for federal judge was almost 30 years ago) that it’s one of those situations where the court can issue a subpoena to someone who isn’t a party to the case, then if the non-party objects to the subpoena, the court will hold a hearing to allow the non-party to present its objection.  So I presume that sometime soon, Apple will get to try to persuade the court to reverse the order.  I’ll be watching for reports on that outcome.