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.
RE: 5 Interesting Investments Mined From the Upcoming Russell 2000 Additions