I was able to contact David Main and talked to him about Drop Zone 4. Here’s what he had to say about it:
“Interestingly, the company I was with, Meadows Games, forbade me from putting what I called The Pacifist Option into the arcade version’s user instructions… it was too controversial. Metaphorically, it was actually much more difficult to win a free game by dropping all bombs and missing all the crudely graphic “ships” than to hit at least one. I thought it was cool to have the feature, and designing the game as I did right after graduating from Cal Poly where I had been neck deep in anti-war activities, I felt a moral obligation to provide a reward for destroying nothing.”
Neither DZ4 nor its ‘sequel’ Bombs Away (1976) have been emulated in MAME yet, so there’s no way to actually test this out in person.