I don't see why that would be true. This is classical signal transmitted optically, not quantum cryptography. There is no reason you couldn't splice the cable through a machine that recorded the signal and then recreated it. Or a beam splitter that removed just a small fraction of the signal; the effect would be a slight increase in transmission loss.
I had a professor who did optics research sponsored by the NSA. They didn't tell him the intended application, but he suspected it was to tap optical fibers by evanescent wave coupling (see http://en.wikipedia.org/wiki/Evanescent_wave#Evanescent-wave... ). It's as if photons are quantum tunneling out of the fiber, so there is no need to physically cut into it. That would have made the tapping nearly undetectable.
The prevailing theory is that they do the latter; the former would be both easy to detect (at the time of splice) and locate (via TDR).
Getting the signal out of the fiber pales in comparison, though, with the task of getting all of that data back to Maryland/Utah.
Unless they have specific cooperation of the cable owners and can tap/split the fibers at the landings, they must be spending a significant percentage of the cost of the original fiber runs (in parallel cables to return the tapped data). The mind reels.
I don't know much about undersea cables (although it is a fascinating subject), but I imagine there's no need to tap a cable in the middle of the ocean. If instead you tap it a couple miles offshore (even tens of miles), suddenly have a lot less undersea fiber to run. And if multiple cables come ashore at the same place, you can probably disguise it as just another fiber.
Of course, this requires a friendly country at one end of the cable, but that's probably not too big a problem.
