Why on earth is it acceptable for the NSA--or any other foreign state's intelligence services--to know the
internal dealings of foreign governments?
You're acting as if that isn't something that should be questioned, and should just be accepted as a general good in both domestic and international policy.
Many of us do not think that such a position is tenable or acceptable, much less a proven and legitimate point of view.
The mission statement does not end with "the Nation". It ends "and our allies under all circumstances." America's allies have a legitimate reason to complain their interests are not being served, and might actually be undermined and trespassed by the NSA's actions--just as America would claim the same thing whenever news reports proliferated with a preponderance of evidence detailing how Germany's intelligence services were tapping the White House phones and slurping up all congressional emails, text messages, and voicemails.
All sovereign states--be they allies or rogues--have a legitimate claim to and expectation of privacy where their internal dealings are concerned. This claim and expectation is even higher among allied nations, among whom it is expected that internal dealings concern other nations will be shared via standard diplomatic channels. And the determinations are left to the sovereign state itself regarding what, if any, internal dealings are shared with allies. But the allies do not possess some automatic right or privilege to spy as widely and deeply as they technologically are able to.
That espionage occurs, that states spy on one another in accordance with their relative positions of power to achieve their own self-interests is not up for debate. Nobody in any of these revelations is suggesting it does not happen. Hell, in the purely functional domain of international relations, there are plenty of net positives that result from espionage being neither endorsed nor prohibited by international law, and mutually allowed and expected among foreign states--such as verifying compliance with international obligations, and confirming the veracity of and commitment behind assurances given by foreign states.
But that still misses the more subtle point--foreign states are well within their rights (and, it might be argued, their sovereign responsibilities) to loudly grandstand and object to details that leak out regarding another state's espionage activities. Why would we expect anything less? That's simply not sensible. There can be legitimate outrage expressed over leaks of espionage because it could actually come as a surprise the depths to which a nation goes when spying on its allies. There could be other cultural and sociopolitical shifts among allied countries in one area, such as the EU, who have in various other ways been engaged in a long process of protecting civilian privacies and, in such a setting, to not engage political theater by expressing contempt for the actions of a state such as America--who shows such blatant disregard for personal privacies and who is caught within their spy's nets--violate the publicly proclaimed principles that the citizens of foreign states hold dear. If the leaders do not object, they could face political backlash.
Nobody doubts that all states engage in espionage against their neighbors, friendly or not. But that does not mean that the release of details showing a much heavier spying apparatus is deployed by the power with the greatest hegemony and strongest power position isn't going to make its allies feel very uncomfortable.
Beyond that, what's most alarming is that Americans truly do not give a fuck. Citizens of other nations, however, do not think like Americans. They've been working on the alliance and diplomacy thing for centuries. They approach things differently, having long learned many lessons through the rise and fall of empires.