Sorry to refer again to my Princeton lectures, but this question really does call for a longer answer. Generally speaking, I report on something when it raises a substantial question of public policy or legal interpretation -- should the US government be allowed to do this, as far as its own laws or citizens are concerned. When the targets are hostile entities and I would destroy the operations by disclosing them, I don't.

Here's a thought experiment. Deliberately exaggerated, but not completely different in kind from the things I'm holding back.

Suppose the files reveal an NSA operation that has planted thought-reading earrings on the mistress of the Martian emperor and discovered his evil plans to conquer Earth. If I published that I think most readers would say (1) that's amazing - I didn't know NSA could do that, (2) I'm sure glad NSA is doing that and (3) it's too bad you revealed it because now the NSA can't do it any more.