Dissatisfied with overbooking

Today I was totally disgusted. I took a family member to Lanseria who was booked on the 14h00 flight to Durban. An sms was received to say the flight was delayed to 14h25 and that check in will close at 14h00. We arrived at 13h30 and he was told with a number of other people that although they had booked there were no seats as the flight was overbooked. We were told that this is normal practice which is totally unacceptable as there were many irate passengers who your poor airport staff have to deal with. These are passengers who have paid for their flight and told sorry you have no seat. It is totally unacceptable that a person has to pay extra to prebook a seat and we have never experienced this before and we have used Kulula a lot. This is of great concern as we have booked family to fly kulula to Durban on 21 March and back on 25 March and I have booked for myself for 26 April to 2 May and I don't see why we are forced to pay extra to get a guaranteed seat. Surely if the flight is fully booked it is fully booked and all seats full y paid for so why is there a necessity to overbook flights. This kind or practice should seriously be looked at because it does not fit in with good service at all. In fact it is totally disgusting and I expect a reply and not some stupid excuse. If people miss a flight that is their loss not yours. Very dissatisfied customer.