I'm pretty sure the government can and does tap any line they want. If you think a private company owning the last mile somehow makes you more secure than if the government did, you're being extremely naive.
Well, I was less worried about the lines and more worried about the described centralized clearinghouses.
That said, although obviously the government can show up with guns wherever they want and tap whatever they want, I suspect the legal situation would be much different if we're looking at government-owned infrastructure vs. privately owned infrastructure.