I think the difference from a psychological point of view is that getting a drone drop randomly on your head feels, well random. Therefore it does not feel like you can do much of anything to avoid it, unlikely though it would be in practice. Getting killed by a terrorist or mass killer feels equally bad because it is random.

But something like sky divers getting killed does not bother me because I can avoid it, by not sky diving. Just like I can choose not to drive and so avoid almost any chance of being killed by another driver. It is the randomness and the fact you cannot avoid it that, I think, makes to less acceptable.