As an employee of another part of the DoD, you can bet your ass we're answerable to congress, and not just for budget. There also happens to be this pesky little thing called the constitution. I'm not sure of the details (I took an oath to uphold it so long ago), but I think there may be some directive in there about the right of people to remain secure "in their papers", and it doesn't give any exceptions (eg, to future government agencies or to corporations).

Actually, it does limit the need for Warrants to "unreasonable searches and seizures".

But, ah! Is this a "reasonable" search on the part of the IRS? Congress thinks so, but we would have to leave it to the Supreme Court to decide.

Both the IRS and NSA are part of the Executive branch.