The lack of a precedent in American history.

As Apple and several security experts have argued, an order compelling Apple to write software that gives the Fbi access to the iPhone in question would establish an unsettling precedent.

If Apple alters the security model of future iPhones so that even its own engineers' "reasonable assistance" will not be able to crack a given device when compelled by the government, a precedent set in this case might lose its lasting force.

The order essentially asks Apple to hack its own devices, and once it is in place, the precedent could be used to justify law enforcement efforts to get around encryption technologies in other investigations far removed from national security threats.