Re: One law for us
My understanding is that the law prevents US intelligence agents spying on US citizens. RAF Menwith Hill (the "RAF" part of the designation is important here) strictly speaking isn't a US government establishment, it's British, and so can gather data on US citizens. Nothing in law prevents US intelligence agents acting on data provided to it by a foreign power.
....or something like that.
(and I believe there's some sort of reciprocal arrangement available for UK to get info on UK subjects from the US)