An FBI inspector general report from 2014 had disagreed with the memo's terms, suggesting that the bureau doesn't believe it's bound to those limitations.

To some extent, Twitter is using the letters as bargaining chips. It's still embroiled in a lawsuit against the US government over rules that restrict how and when it can disclose data, and the revelations support its belief that it can't offer "meaningful" transparency to users. How can you talk about examples of law enforcement overreach when you're not even allowed to acknowledge that a given National Security Letter exists? We wouldn't count on the court deciding in Twitter's favor, but it at least has some supporting evidence under its belt.