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.
He claims that the FBI already has all of the suspect's communication records as retrieved via service providers, but ignores the important detail that iMessage uses end-to-end encryption, rendering any such records unobtainable by the service provider.
Yes, the last backup was six weeks prior to the phone being seized, but this only means that the phone may well include six weeks' worth of pertinent evidence. And there is nothing to suggest that the FBI is only interested in messages between co-workers.
Ultimately, the phone's data would need to be decrypted and analysed to see if the first four assumptions that Snowden makes are actually true.
The final assumption is that there are other feasible technical measures that could be taken to crack this phone. This would perhaps be the most interesting point, but Snowden chose to not expand upon it at all.