I don’t have his documents on hand, but I’d instead recommend you read the transscripts of the NSA-Untersuchungsausschuss of the German Bundestag, which investigated the NSA and BND affairs. They’re official government documents, and are transscripts of people testifying under oath.
The head of the BND stated that the BND did not record any information about Germans, and the NSA did not record any information about Americans, but that the BND shared whatever it collected about Americans with the NSA, and in reverse.
That agreement is to not to spy on each other against each other's will. That is completely different from spying on each other's domestic targets at each other's request/will to workaround domestic no spying and privacy protections laws.
UK spying on US citizens doesn't violate US Constitution and the 4th in particular which naturally applies only to US government and the people on US soil. And thus they do it that way. UK obviously has all the rights to listen, record, etc on telecommunications of all of the foreigners, including US people on US soil as long as the traffic leaves the US (and even if not - i don't think they bother to pay attention to such a small detail as long as they are doing it from comfortable office in London :).
>Explain how anyone would think that works.
US government lawyers explained how torture is legal. Compare to that, the circumventing of the 4th by means of allies doing the actual spying ... i'd be surprised if they even bothered to explain it away.
Officials of the BND admitted it in a parliamentary hearing, too.