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.
Let me tell you about this new product of mine. It's called Rhinoceros powder, and it does - you guessed it! It scares away Rhinos. As long as you spread Rhino powder everywhere, you will not be attacked by a Rhino. It works particularly well in urban areas.
It clearly works, too! I've been using it for decades, as have many of my customers, and wouldn't you know it! None of them have ever been attacked by a Rhino.
I'm not sure if it's intentional, but there was a Simpsons parallel in "Much Apu About Nothing":
[Later, a full-force Bear Patrol is on watch. Homer watches proudly.]
Homer: Not a bear in sight. The Bear Patrol must be working like a
charm.
Lisa: That's spacious reasoning, Dad.
Homer: Thank you, dear.
Lisa: By your logic I could claim that this rock keeps tigers away.
Homer: Oh, how does it work?
Lisa: It doesn't work.
Homer: Uh-huh.
Lisa: It's just a stupid rock.
Homer: Uh-huh.
Lisa: But I don't see any tigers around, do you?
[Homer thinks of this, then pulls out some money]
Homer: Lisa, I want to buy your rock.
[Lisa refuses at first, then takes the exchange]
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.