IANAL, but it appears to depend under which legal authority the letter is issued (Verizon's was 50 U.S.C. 436). See the chart on page 15: http://www.fas.org/sgp/crs/intel/RL33320.pdf
The chart of page 15 of the document you linked says a 50 USC 436 covers "all financial information relating to consenting, identified employee" but wasn't it used to obtain customer information from Verizon?
What am I missing here? Is what the document is describing just one possible use case?
Edit: typo
You're right; I misread it. It's 50 U.S.C. 1861. Sorry, I'm out of my depth.
Lol so am I that's why I wanted to clarify. Thanks for clearing that up