If they are smart, rational people, you can explain that the dog doesn't just go out and get to bite whoever it wants. There is a lot of technique, focus, obedience, and control taught to the dog before it ever gets to go after the "bad guy." It is good to teach a dog when it can bite and, more importantly, when it cannot. And to many of the dogs (mine is definitely one) it is a game.
If they are not as rational in their thinking, explain it like a person taking a kickboxing class. Just because you learn this skill doesn't mean you're going to go attack someone. The class teaches form, technique and skills that you could use for violence, but hopefully will only use to make yourself a healthier, happier person.