Why would the code be unsigned? If Apple wrote the backdoor OS, they could presumably sign it.

I incorrectly totally misread the OP and thought was talking about FBI flashing it themselves, without Apple help. Yes, of course Apple can sign it. I stand corrected but can't delete my comment.

To clarify, I agree that nothing they ask of Apple is technically impossible or even that difficult for Apple to pull off, probably via simple DFU without touching the flash at all.

Hence the need for a validbOS from Apple.

why load unsigned code? can't apple sign it?

Or anyone else with the apple key such as the NSA.

The judge wouldn't need to ask Apple if that were the case.

Or the NSA doesn't want to reveal that they have Apple's code signing key.

True, but a judge wouldn't allow the NSA to decrypt with a stole key either.

That wouldn't be valid evidence in court.