I agree with your general thesis. The "And once you come up with that answer, how would you know if your giving those things actually was the thing that reduced it?" seems "simple" enough in that it would result in a number of arrests involving the methodology of surveillance in question. You couldn't say they wouldn't have been caught otherwise but you could at least say "X has produced Y for these cases." no?
No. Because 1) X itself may enable Y which might not otherwise have occurred and 2) since resources are finite doing X to yield Y means that you couldn't do Z instead which you might value more than whatever utility you got from Y.
