I believe you that the NSA might provide intelligence to its partners about foreign nationals that those partners could not lawfully collect themselves.
I'm wondering if NSA can actually procure intelligence about US citizens from its partners.
It's a well established rule that private party's can hand the U.S. government information of there own free will without issue. The issue is the government can't request specific information, but a wink wink system of bulk data sharing should be perfectly legal.
Beyond that point, I have no idea if there is some exemption that lets them request specific information.
Nowhere in that article does it appear that GCHQ actually asked NSA to tap UK citizens' private communications. Is it buried under one of the other links?
Instead it appears that NSA had gathered data relating to UK citizens (under NSA's existing legal authorities) and that the GCHQ then asked for that data to be shared with it.
UK law is incredibly surveillance-friendly compared to the USA so I'd be surprised if there were even a reason for GCHQ to ask NSA to do surveillance on GCHQ's behalf; GCHQ could much more easily do it directly.
But NSA-sourced data could still be useful for GCHQ, which would make sense that GCHQ asks for it when it's been collected, for the same reason Germany's BND was found to be sharing data with NSA (in BND's case, because they wanted NSA to share their own data with BND).
Note the actual judgement is linked on the bottom.