Google and Amazon have their own dedicated fiber between DC sites, but there was evidence that the NSA was still tapping it at some points. So they added encryption between the sites as an extra safeguard.
Of course executives at Google may have handed over a set of keys to the NSA after this was done. If I had to bet I would say that the NSA has more than one way to get whatever data they want from any of the top tech companies. Either via stealing, insiders or coercion they have many options.