There are about 300 million Americans.
How does GCHQ know which ones to spill the beans on? It can't simply be all the information as NSA would then still need to choose which ones to look at, which would require a separate warrant to "look", even under their surreal logic.
It can't be a specific person directed by NSA, as that makes GCHQ an agent of the U.S. government.
As with every other time we mention NSA, if you're going to assume they will break the law as a matter of course then you must leave the Internet immediately or face spying. :P
On the other hand if you want to posit that being easy to break the law is reason enough to burn all the transatlantic fiber so that GCHQ can't cooperate with NSA then simply say so, it's at least theoretically a defensible starting point for a debate.