Poor policies regarding flight change

Flights were booked for me by the South African National Research Foundation (NRF) from Johannesburg to Cape Town, and the scheduled meetings ended early. I thus arrived at the airport early, and instead of being allowed an empty seat on an earlier flight, I was told I had to wait for four hours for my booked flight (even though this booked flight travels at a peak time i.e., 6 pm). Personally, I feel that this is poor customer service and also a poor business decision (given that I could fill an empty seat on a plane that is flying any ways, and my 6 pm seat could be resold at a premium). In my feedback to the NRF, I will also strongly recommend that South African tax-payers money be spent on national flag carriers instead of foreign airlines, especially ones that exhibit such poor customer service.