Apple has complied with these legal requests to bypass the lock on iPhones many times in the past (about 70 times, according to
this NPR report). Apple designed security features that are more difficult to bypass in their newer iPhones (using iOS9), and they'd need to come up with new software/procedures to allow them to do the same thing for their iOS9 phones as they've done before, but they aren't claiming they can't do it.
The argument that another government (China, etc) might ask Apple to unlock an iOS9 phone if they do it for the USG, and that this is a reason not to assist in this US investigation, appears fairly weak. Another government could demand that Apple do that
anyway, and refuse to allow Apple to sell phones in those countries if they won't comply.
The argument that Apple shouldn't assist in this case because that will just unleash a flood of questionable requests from other US authorities seems to ignore the fact that these agencies have already been requesting this assistance for years and Apple has been providing it without any apparent abuses or difficulties.
Apple is not being asked to add a new back door to their phones or anything that will make them easier to hack. They are being ordered to provide technical assistance in a lawful US investigation. If Apple develops a technical means to bypass the security measures that they have engineered into their phones/OS, presumably they will have a darn good business reason to keep those tools/methods well secured. Apple has all kinds of sensitive data already (their proprietary source code, customer financial data, etc), keeping this new software and procedures secure would require the same measures they already use to protect this other information.
As a practical matter, dissidents, criminals, terrorists, or just common citizens who want to communicate without being observed by a government probably shouldn't depend on Apple or any other corporation to protect their secrets against requests by these governments ("good" governments or "bad" governments).
IMO, Apple is using this as a marketing opportunity. If they prevail, they have proven themselves to be champions of privacy (never mind their past actions). If they lose, they get to say that they are assisting only after fighting the good fight.
For comparison/background,
here's a fairly concise summary of the types of information Google supplies to US authorities under various circumstances. There's nothing unusual about US companies being compelled to assist in lawful investigations. The degree of assistance requested in the case of Apple and the Farook investigation may be unusual, but that's what it is--just a matter of degree, not new legal ground.
Apple is not being asked to help the US government oppress citizens or illegally spy on people. They are being asked to assist the FBI in getting information off a
government-owned phone (County of San Bernardino) as part of a legitimate criminal investigation. There may be other future cases where their similar assistance keeps innocent people put of jail, prevents future crimes, etc.