A beaker full of death wrote:
And the answer is so that the human, American, 1960s audience will be able to understand, if not relate to, the characters.
In the 60s, the 2 finger peace sign salute was quite popular.
Was this hand gesture always used with the same hand? I think that the generation of the 60s that used this hand gesture could relate quite comfortably with the Vulcan salute without it mattering which hand the salute or gesture was made with.
I know that gesture dates at least as far back as WWII (V for Victory), and might go back earlier than that.
Richard Nixon frequently used both hands in the pose when arriving at a destination to greet well-wishers.