This is trivial - the whole purpose of foreign intelligence is to help the interests of your country while disregarding or actively harming the interests of everyone else. They have no obligation whatsoever to protect (or even refrain from murdering) others, but they do have an obligation to protect their own citizens, so they have restrictions for that.

The only two valid reasons for NSA not to capture all the foreigners email is if (1) it's too costly (and it probably isn't); and (2) the goverment decides that the PR harm is greater than whatever they gain from having all the email (and it probably isn't either).