I'm pretty sure NSA manages to get access to backbones just fine without the benefit of policies that try to expand the role of fibre in the last mile. NSA has ample access to backbone fibre, both at exchange points and undersea (they have submarines). This is an utter red herring.
The conduit would presumably be owned and managed by the government. The fiber in it by the people who pulled it presumably the provider.
But here is the thing. Conduit size is not infinite. If not large enough it could get filled. I can even see a case where a particular company may put in extra fiber just to prevent someone else from doing so (and then either not using or renting that fiber). The government is not particularly good at preventing this type of gaming and manipulation.
Don't get me wrong, I'm all for it, but what are the downsides to this? Where is that explored?