Its data protection.
If Apple unlock this one without a court order that gives the security services some leverage. The NSA could argue that the IPAD they just took off someone needs to be unlocked, and is also a special case, and shouldn't need a court's approval to open.
Many on this thread have used an emotional arguement. It's an emotional case. But you have to step back and think if the implications. Apple are doing just this. They KNOW that they are getting bad press on this, but they KNOW that it can be far worse, in the long term, if they are seen to be an organisation who will sometimes allow data access without a court order.