I think it would be impractical to force every federal agency to put make every legal interpretation public, simply because people wouldn't do it. They'd just avoid putting things down in writing instead. Ultimately, any internal interpretations have to be solidified and put down in writing in any court proceeding. I think that is the backstop against internal interpretations run amok. And it does function as an effective backstop in other areas of government, where every agency action is met by dozens of legal challenges that play out in public courts. The problem with the NSA situation is that the NSA's actions are not subject to regular court challenges, and that's by design. Congress made it that way.