USS The Latest will have all the countermeasures it needs. What about $random_middle_eastern_wedding_guests? Countermeasures aren't available to anyone but the military. But in some parts of the word everybody can be a target, especially if some superpower needs to maintain its sphere of influence, or they happen to be at war with some genocidal regime.

Drones have been used quite successfully for many decades now and clearly haven't developed any countermeasures for what most of their targets are concerned, "quickly" or otherwise. This is why we still read about drones bombing people out of existence, whether the correct target or not.

An autonomous drone just outsources the usual killing from the human operator to a script, making it even more likely to make a mistake and also apply the desired effect: terror.