It's not so much the NSA as various other government agencies. The NSA is hoovering everything up, but if the local cops call them and want access to it, the NSA is going to tell them that they're not even authorized to know whether or not the NSA has that information. Also, something something due process something something American citizens.
Whereas if they can get the telcos to keep it then the cops can get it using the third party doctrine. This is basically an end run around the constitution, which is why they like it.
