Why asked to pay for Emergency exit seats

Arrived at OR Tambo after checking in on line, SA 543...no mention of payment and was told to pay extra for the emergency exit seat. This when no other seats other middle seats available and the check in person did not know how much, so was sent to the counter. I refused to do this and thus ended up in a middle seat after my electronic check in was confirmed for an ailse seat. When I did the same electronic check in in Durban, SA 542 and the system allocated me an emergency exit seat, I checked in on line and there was no question of payment at King Shaka airport. Are SAA getting so desperate for money, they are now trying to rip the customer off for any few rand that they can. Is this only OR Tambo or do they just pick when a flight is full. Both flights were full anyway. I want an explanation for this so I know what airline to fly with in future.