If you have been a victim of a crime and the perp walks, it's reasonable to say something besides, "Thanks goodness our system works to protect the innocent!"

I also think prosecutors should always be a little maddened by it. It's they're job to keep the guilty-but-free error as low as possible.

Or to put it differently, aggregate feces reduction is great, but you are unlikely to say that if you discover a bit of somebody's poop in your mouth. And you never want the person in charge of cleaning the pool to say, "Fuck it, a little poop never killed anybody."