It's a shame this is the top post.
A National Security Letter absolutely cannot do anything like this. NSLs are such a weird HN boogeyman. Absolutely a violation of the first and fourth amendments, but they are administrative subpoenas. They cannot compel the kind of action you're talking about.
There are plenty of ways to twist interpretations of statute to try and make investigative and prosecutorial overreaches legal, but it's ridiculous how often these NSL posts get voted up to "educate" others on a subject the poster clearly hasn't even read up on themselves.