The cities pay for an annual subscription, not per gunshot. So as long as they see a need/benefit then they are in business.
I don't see it as an ethical dilemma. ShotSpotter isn't out there creating gun violence, in fact they are doing their part to reduce it by reporting it to the police quickly.
The way they make their money doesn't really matter, because they would be out of business if there never was a gunshot. So if shooting stops, they run out of business regardless of the way they bill.
And I don't know about you, but when I invest in a business I expect it to increase its sales for the next 10 years. So if I were to invest in ShotSpotters, I would be betting that gun control won't get better at least for the next 10 years. Unless of course there is a preventative effect involved. So that the service acts as a deterrent.
Anyway, seeing as in Finland we have like 5 incidents per year where someone shoots a gun where they aren't supposed to, my perspective might be different. Not that either perspective is wrong.