[dannyw]

Of course they could, but then you'd know it's the US doing it; with cryptographic evidence.

If there are covert operations going on somewhere in the world, where the US isn't publicly claiming to be (and I don't just mean DoD; but also CIA for example); this would be an irrefutable cryptographic trail.

Another case is unintended collateral damage; e.g. say a civilian airliner (with US citizens perhaps) goes down; the US can't have plausible deniability and suggest it was the enemy doing it.