[PeterisP]

For a few (IMHO realistic) examples:

* the "splitter" as you call it (an useful simplification for this discussion) might be on devices of public companies without their consent against their wishes - IIRC that was part of Snowden's revelations.

* the "splitter" might have been secretly placed on an undersea cable in foreign waters, and there's a clear national security interest not to reveal that fact - the key job of NSA is to listen in to other nations and ensure that they don't know if/how it's succeeding.

* the "splitter" might include cooperation from friendly governments that's not public knowledge to their populations. Again, there's a clear foreign policy interest not to disclose that.

* the "splitter" might have been made possible by a vulnerability in something that everyone believes is secure (e.g. RSA cryptosystem), and revealing what kind of data is captured will reveal that it's possible to capture that kind of data, which (if it's a surprise) by itself might lead to it becoming useless as people worldwide deduce what component needs to be replaced.

There's obviously more. NSA can reveal that their job includes spying on everyone outside USA; but any more details would generally be harmful to that mission.