If Apple couldn't do an end-run on the key derivation protection mechanisms, then they couldn't use that backdoor to aid the FBI.
"Can't do it" is a strong defense.
"Won't do it" isn't.
e.g.: http://daringfireball.net/linked/2016/02/17/panzarino-apple-...
It's not as easy as you imply. Denying themselves the ability to end run your security might well require them to not be able to upgrade or recover user devices. At all.
In general, Apple's guarantees about anonymity and security all assume that there is no risk to Apple sitting in a trust position. This case demonstrates why that's a bad idea.
This isn't some NSA mass surveillance program, or clipper chip reloaded(TM).
This is an active murder (and terrorism) investigation in which Apple's privileged position -- one they assumed by choice and to their own benefit -- already exists. It is not unusual, unethical, or establishing any new precedent for them to be compelled to use keys they already have to open this single device.
Apple does not need to, nor are they being asked to, provide a backdoor that can be used elsewhere, and any further demands of Apple will require the same judicial oversight as this one.
If Apple doesn't want to be in this position in the future, they can avoid the potential liability by not holding master signing keys that can be easily used to circumvent the security of an owner-locked device.
They are, however, being asked to _create_ a backdoor that can be used anywhere. The specific request is to remove the limits on guessing passcodes so that the FBI can brute force it.
All they're being asked to do is to use it, entirely under their own control, for this one phone.
Tim Cook is trying to pretend that commenting out a few if statements in the key derivation code path is what creates the backdoor.
If Facebook can't read your data, then they can't sell your eyeballs to advertisers.