I don't have any real data or direct experience to back this up, so I'm really just pulling this out of my ass, but the NSA seems like a very different entity than "law enforcement" when it comes to privacy.

The rules, laws, hiring, and budget at stake is very different at the NSA versus your local FBI or police office. Think ten or eleven figures versus six. Polygraphs and M16's. Football fields of data centers.

Frankly the NSA can read my email. Law enforcement is a whole different beast.