But hey, the FBI exists to turn over every stone. The real story here is that they specifically chose this case to go after Apple because they suspected the backlash if Apple tried to fight it would be more than they could handle.
The FBI's request isn't too bad really. It's not like they are requesting Apple to build a new mass surveillance tool or anything similarly far-reaching. This is limited to the domain of digital forensics, and how best to extract data in a criminal investigation, against firmware that is hostile to cracking. Given that, by design, the phone has been locked down to only accept Apple-signed firmware, then Apple is really the most feasible choice to assist in bypassing this.
Apple is the only entity who can reliably alter the firmware to remove the forensics-thwarting restrictions, due to updates requiring a chain of trust all the way up to Apple's root certificate.
The physical device may not be owned by Apple, but the firmware installed upon it certainly is, both legally (due to copyright law) and cryptographically.
This is an interesting observation, actually. It suggests that there is a (legal) price to be paid for retaining tight control over firmware.
After 5 minutes of search I cannot find the story. But there was a super interesting story (in The Atlantic maybe?) about an investigation into an assassination plot where the major break was exactly this slip-up.