One could assume that Americans are spied on by foreign governments and the data is just exchanged. The US spies on Brits, the UK spies on US persons, and the both compare notes.
Actually this is exactly what occurs. Intelligence exchange among America and its allies under Echelon, ANZUS, and UKUSA have been used in this exact way to end-run around anti-domestic surveillance laws.
This is exactly why "every country spies on foreigners" is unacceptable. There's no reason to assume that this "note-sharing isn't already taking place.
I know NSA's mandate is to spy on foreigners, but it's still very jingoistic and xenophobic that not being American makes it OK to spy on you.