Apple responds to the courts order on two primary fronts.

First Amendment Violation
Compelled software code and code signing is “…compelled speech…in violation of the First Amendment”.

Fifth Amendment Violation
“…conscripting a private party…to do the government’s bidding…” violates Fifth Amendment rights

Article on Motherboard along with copy of Apple’s Motion to Vacate filing with the court.  I’m not a lawyer but two points I find interesting, 1) software code and signing of software coding is argued as protected speech protected under Constitution, 2) major corporations have constitutional rights just as US citizens do (I didn’t realize this).