Exactly. Why would anyone want to do that? I don't think they are seeing a dentist for this procedure so I can't imagine the risk for infection and complication.
Why? Why does it bother you? Let people live their lives how they want. If you don't like it, leave them alone. Don't associate with them. As long as they're not hurting you, it shouldn't matter. There are billions of other people in the world you can associate with.