I remember this with the Clipper Chip, and FBI Director Freeh. It is understandable that they want this -- makes their job a lot easier, and makes a lot more material to sift through.
However, there were the same issues with this wiretap stuff as with the Clipper Chip:
1: Bad guys getting access to the backdoor, just like back then, bad guys getting access to the LEAF (law enforcement access field, part of the key escrow mechanism.) When (not if) this happens, every single endpoint is wide open, and this becomes a national security issue when companies start getting hacked wholesale and there is nothing they can do except power off and unplug.
2: Abuse. Of course, this would allow anyone with access to this a lot of material they can scoop up, and sell.
3: There would be -billions- spent by rogue nations, criminal organizations, and others to get at those master keys. When the money is at stake, it will turn into a game of finding out what people are even close to the master keys, and kidnapping their family. The billions spent on compromising an update repository in order to get backdoored programs into the target would reward the rogues with trillions.
Securing the master keys is one thing. Keeping them secure while in use for massive eavesdropping and protecting them from leaks is a very difficult task. Someone in the chain can be compromised eventually, which leads us to point #1.
Plus, we already have a shitload of ways that an endpoint can be compromised. A lot of software updaters send a unique computer ID. It doesn't take much to have a certain ID get a slightly modified signed update while everyone else gets something else.