Kulula overbooks and overcharges, exploiting pensioners

My mother had a return trip booked from Johannesburg to Durban. The day before she was to leave, we checked her in online, selected a seat and printed the boarding pass. We arrived at the airport long before boarding would start. She only had one bag to check. To her dismay she and 9 other passengers were told they could not be accommodated and would need to wait for the next plane and pay an extra amount of R900!!! What is the purpose of the online check in and seat selection if the airline overbooks the flights? Truly disappointed. She is a pensioner, she should be treated this way.