The other dangerous thing about this attitude of, oh, it's only foreigners, is that GCHQ and other US allies routinely sweep up communications from all over the world (in the case of GCHQ probably mostly from the US). So if you send information to Europe from the US, you're being spied upon, and the information relayed back to the NSA. The same goes for citizens of the UK subject to NSA spying who have data or contac…
So then this argument that "we don't spy on our own people", implying that "we have no data on our own people" is potentially misleading to the extreme.
Of course, NSA already spies on Americans, but other countries (Canada, Australia, UK, and others) might not be so "brave", and they could be getting data about every one of their citizens from NSA, which makes it just as bad as themselves doing all the spying. So the "legal boundaries" of these spying agencies are effectively useless.