Did you read their motion?[1] It is very much part of their argument. It sets a precedent that not only will other US courts compel them to unlock devices, but other countries will as well. Even if you trust the US government to not use the software maliciously, do you trust other governments?
What's stopping other governments from compelling then to unlock phones now, given they've effectively said they can.
What's stopping rogue employees from selling that capability - is Apple Computer's internal security good enough to see off espionage by nation states?
They've already asked and Apple has had a strong bargaining position.
"Beijing last year backed off on some proposals that would have required foreign companies to provide encryption keys for devices sold in the country after facing pressure from foreign trade groups."
1) https://www.documentcloud.org/documents/2722199-5-15-MJ-0045...