PRESEATING CHARGES R250 PER PERSON - *******

We booked flights via Travelstart to travel in the USA - we have 7 flights booked 2 International and 5 domestic. The domestic flights are with United Airlines - we have managed to book our seats for travel in June and July this year. I attempted to pre-book our seats on SAA for the 2 International flights today and was told its R250 per person additional charge to book our seats.... What a *******...... why are we permitted to book our seats way in advance with other service providers but not SAA...... We travel overseas often, I will ensure I avoid SAA in the future...... When I asked customer service why we were charged to pre-book seats, she said it was SAA policy because we were travelling economy class.....