The FBI is only asking apple for 3 things:
"it will bypass or disable the auto-erase function whether or not it has been enabled" (this user-configurable feature of iOS 8 automatically deletes keys needed to read encrypted data after ten consecutive incorrect attempts)
"it will enable the FBI to submit passcodes to the SUBJECT DEVICE for testing electronically via the physical device port, Bluetooth, Wi-Fi, or other protocol available”
"it will ensure that when the FBI submits passcodes to the SUBJECT DEVICE, software running on the device will not purposefully introduce any additional delay between passcode attempts beyond what is incurred by Apple hardware"
While I have firm beliefs in every American citizen's right to privacy, the San Bernadino lost his privacy rights when he took another innocent individuals life. Any information that is on his phone that may bring peace or clarity to the situation should be brought to light, and Apple should not be able to stand in the way of that simply because they manufactured the phone.
Considering the fact that he was a terrorist, his phone could contain some information that could help the FBI in preventing future attacks. This is an exceptional case and it surely would not affect the security of iPhone in general. Not allowing the FBI to unlock the attackers phone means that he gets some kind of protection from Apple.
No, that would be very dangerous precedent and it would be only a matter of time until FBI asks some other company to give them the keys to their products and, more importantly, their user's data. Not to mention the damage to the Apple's reputation (already stained by the iCloud being hacked and celebrity photos leaked) and potential financial loss.
The Federal Bureau of Investigation is trying to force apply to write a backdoor into its operating system. Apple on the other hand has gone up in arms and tried to stop the FBI from breaking and entering. A zone of privacy created by the Bill of Rights protects the information on iPhones because for one it is private property and two it is an extension of our brain and cannot unlawfully be searched or seized.