I don't know for sure but when I thought about it this makes sense.
I would assume that a right handed person would need to use that hand too write down messages recieved, So in a "conversation" use the left hand to send, right hand to recieve, make sense?
I don't think this is likely unless the operator was ambidextrous. Transmitting Morse depends on a sensitive touch. During WWII many experienced operators could recognise each other simply by their "touch" on the key. This was like a signature & no 2 operators were the same. It would not be possible to transmit as quickly or accurately with the "wrong" hand.
