This sounds like it may be in violation of CIA charter. If memory serves, only the NSA is precluded from spying directly on American citizens in these United States. The CIA, on the other hand, is not thus handicapped.
NSA and CIA are indeed so prohibited. However, there is no prohibition at law against NSA/CIA/USwhomever listening in/monitoring citizens of the UK ... and similarly there is no prohibition at law against MI6 listening in/monitoring citizens of the US.
This curious little legal quirk has matured over the years into Echelon and related systems.