Really what it is is they had a lawyer say, "We can justify massive spying on citizens via [long long list of arguments and precedents]", the security apparatus would like to keep the veneer of legality that provides and knows if what they were doing was known there would be massive backlash and the law would be changed to make it more clearly illegal so they keep that interpretation secret. Basically going on the logic that they don't have to tell Congress or the Courts how they screwed up the 4th Amendment and it's not their job to fix it.