1. We don't have a right to privacy.
2. The power of the constitution ends at the border.
It's pretty sick, but that's what it amounts to. The CIA can't operate within US borders but it can operate at and outside borders. That means sending messages internationally are fair game for warrant-less searches.
That doesn’t explain why lawmakers would allow their own government to (indirectly) spy on them. Or are they so full of integrity that they would say “I must be spied on as well as my constituents, you know, for fairness”? /s