As I understand the AAG's position, he wants to be able to serve a warrant on a third party (the hardware/software company) to get access to information. Right now, there is a mechanism to compel defendants to comply with a warrant (contempt of court). But it isn't 100% effective, and trying individual cases is time consuming. Why serve individuals when you can serve tech companies and get lots of cases resolved at once? It also eliminates that pesky problem of individuals challenging the warrant instead of putting a gag order on the warrant so the subject never knows until they are hauled into court.
We have never required safe manufacturers to create a master key for law enforcement. Why should electronic devices be different?