That's a fairly naive argument. My local bank discourages potential robbers with armed guards, alarms, and a locked vault. Just because they've never charged anyone with bank robbery doesn't mean those measures didn't prevent potential robbers from deciding to become actual robbers.
It's really not any better than the argument that of course the TSA keeps away terrorists and tigers because there haven't been any hijackings or tiger maulings on airplanes since they took over.