>Ranger John Reid might take exception to that, but Tonto would
>probably point out that they do, in fact, have a silver mine, so
>Kemosabe should just shut up and nod.

The in-universe explanation originally was that it was a reminder to him of how expensive a human life was. That fell by the wayside when it became a good way to authenticate his identity without a Lens.

Michael C. Fortner
"Maxim 37: There is no such thing as "overkill".
There is only "open fire" and "I need to reload".