Even if we could trust the US government, we shouldn't be forcing Apple to create this 'master key' anyway. The problem is that anything that has been created digitally cannot be destroyed after it is used. Once the tool is created, we cannot put that genie back in the bottle.
The entire point is to let the genie out of the bottle.
By having a device ID check (combined with digital signage), Apple could publicly release the tool without fear of anything happening on any other phone. Plus people couldn't modify it because that would break the signature.
Also, if you think having a device ID check is going to do anything, you might want to have a look at the cracks scene and what they do with your pitiful attempts at DRM...