They do it that way to confuse the Russians!

The reason it's labeled OFF-R-L-BOTH is due to the nature of grounding. When you turn the key, you're connecting a circuit; when you turn to R, you're completing the circuit (AKA connecting the P-Lead and grounding the left mag). This is in the left position, and the manufacturers logically chose to display which magneto would be running in this position instead of the one that is grounded.