Just curious, but why are you saying it was legal? We can all say one way or the other. What's your argument? It's quite easy to say something is "either legal or illegal". But there's a lot of grey area in the law, and many times that's what government is counting on.
If you avail yourself of any benefits of the public credit system, you waive the right to challenge the validity of any statute pertaining to, and conferring "benefits" of this system on the basis of constitutionality. The United States, as a Chapter 11 bankrupt estate, operates on credit obtained by the executive branch of the Government of the United States. Every federal agency operating under the protocol of USC/CFR is active in servicing the US debt to its creditors. All people, as U.S. citizens, are exposed to this system, thus they waive their constitutional rights in affairs of "interstate commerce." Too obscure? Think about all of this in a financial context, and follow the money. Spying on citizens is done on basis of financial impetus.