Earlier quoted context omitted.
Perhaps because foreign leaders do not consider tapping the phone lines of our allies to be within the scope of NSA's stated mission "to gain a decision advantage for the Nation and our allies under all circumstances."[1] Or maybe because SigInt is supposed to be working "for intelligence and counterintelligence purposes and to support military operations."[1] They might not get why tapping their phones supports Amer…
How on earth is knowing the internal dealings of foreign governments not "gaining a decision advantage for the Nation"?!
For example, releasing a genetically engineered virus which killed the entire population of the South American continent would gain a decisive advantage for the US nation, because it would open up lots more lebensraum and could be invaded and settled, but it's not usually seen as an acceptable way to behave.
Unacceptable actions include cheating on your allies, treating your enemies like animals, massacring civilians in or out of wartime, etc. Cheating on allies is particularly damaging because it means no one will trust an American from now on.