Lack of knowledge over ... signing
This is a general problem. Although a secretary of state might be theoretically "responsible" for the decision, in practice they're not going to read everything that passes over their desk. If they do read it, they'll only know what they've been told by officials and in the rare event of asking a question, they're only going to be able to ask questions based on the selective information they've been given and will likely get selective answers in response. The notion that "an elected politician makes the decision to authorize an investigation rather than a civil service official" is purely theoretical, but it's a great form of absolution for the officials involved.
Edit: As the letter clearly shows, the alternative of having a judge approve warrants is of little use if there is a ready supply of retirees from the court of appeal who don't feel the need to keep up with the law or, presumably, rock the boat.
Biting the hand that feeds IT
