Ace of Spades explained how it works:
Remember the US intelligence trick?
They can't spy on US citizens without a warrant sufficient to clear the probable cause evidentiary hurdle. But if they tell foreign intelligence services to do the spying and slip the results back to the US intelligence community, they get their spying done by proxy, avoiding a judge's scrutiny and any intersection with the US Constitution at all.
This is where it started - years ago. Years before anyone knew the implications of these tactics. The Brits could feed back to the thugs in the CIA all they wanted to know. It was OUR CIA finding ways around OUR laws. That's the most telling... and the commies? Our info was so spread around they go their pieces too..