I like the way it worked before the Patriot Act: want to search someone's property? Get a warrant. Want to tap someone's phone? Get a warrant. Want to collect whatever private information? Get a warrant with the target's name on it.
Although that said, they read ALL emails, and have said [0] that they require no authorization or warrant to listen to your calls, read your text messages, and read your email. So no, without explicit reigning in, those things the GP lists are not all as they once were.
[0] http://www.cnet.com/news/nsa-spying-flap-extends-to-contents...
From 1946: http://en.m.wikipedia.org/wiki/UKUSA_Agreement "For example, the British newspaper The Independent reported back in 1996 that the U.S. National Security Agency "taps UK phones" at the request of the British intelligence agency MI5, thus allowing British agents to evade restrictive limitations on domestic telephone tapping.[53]"
Note the actual judgement is linked on the bottom.
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.
Beyond that point, I have no idea if there is some exemption that lets them request specific information.
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).
Which uses 53 as its source. "US spy base `taps UK phones for MI5'" which may or may not be credible. http://www.independent.co.uk/news/uk/home-news/us-spy-base-t....
However, my second posts links to a court case which suggests they overstepped some bounds.