Because he's dead he technically has no rights. What's at issue is the government requiring lock makers to make a pass-key for all the locks in your house. It wouldn't be long before those keys get out and locks became useless. Does this make us more secure since Law Enforcement mostly does not do interdiction.The content on the phone could change his sentence from death penalty to life inprisonment. This is important business.
Imagine the damage of Bad Actors being able to penetrate anyone's database. Easily learning things like people's secure routes, codes to alarms, information to force blackmail, etc.