Seat changes

I booked a flight from George to OR Tambo (CNPDYV). Booked on 16/2 and paid for seat 16D. When I printed my boarding pass on 19/2 the seat had been changed to 1A. When I got to the airport the seat had AGAIN been changed to 13D. I stopped flying Kulula as this has happened to me on numerous occasions. What is the point of paying for a seat and Kulula just changes them as they see fit. Very annoying, why bother booking and paying for a seat, or for that matter flying with Kulula!