The other side of this is, "What's an acceptable number of crimes against children?" Implicitly, there's a choice, "This remedy is projected to reduce crimes against children by X%, but we're not going to do it because Y." The projected marginal reduction is justified using the consequences of Y.
I'm perfectly ok with this calculus. If someone wants to say that right to privacy is more important than a projected reduction in crimes against children, more power to them.
What I'm asking for is honesty regarding this calculus. For some reason it short circuits peoples' brains and the consequences of taking whichever action end up making them unable to say, "Yes, the [projected] marginal reduction is the what I'm willing to spend by not doing Y." Just say it.
Any thoughts on the use of prediction markets, especially ones where predictor performance is tracked, in order to make better predictions on the results of legislative action?