[openasocket]

Two thoughts:

1. The public doesn't know, but the same may not be true of US intelligence services. There are various ways to drop hints. Have a colonel pretend to get drunk at a bar and ramble something about drones. Feed a known mole some hints about an operation underway. Maybe a unit stationed on that base recently was involved in some operation the foreign adversary didn't like, and they have a diplomat cryptically say something about reprisals. But most importantly you do it in a way that it isn't definitive proof, or publicly accusing them would require revealing important sources/methods. That way the US can't publicly respond or escalate.

2. Perhaps the goal isn't necessarily deterrence, it's misdirection. Reveal the vulnerability of civil and military infrastructure to attack in a public and embarrassing way, and the US government will be required to take actions to prevent this from happening again. And it's way more expensive to secure infrastructure than it is to attack it. You could get the US to spend billions on securing this infrastructure, which is dollars that aren't being spent on something that could be used offensively.