Legal Theft?

So. Here I am fighting with Flysafair again. Made a booking for me and the wifey. Well she made the booking. I have to unfortunately miss the flight due to work responsibilities. Now they reckon that the money for my ticket, 1x Adult Return flight to Capetown is non refundable. They can not provide me with a voucher. The flight can only be moved to a future date...which costs an additional R600.00 So....if I choose a future date,any date, if something comes up like work. Or death. I will have to pay to have the flight moved again. I call this theft. Straight up theft. And the supposed manager reckons thats just the way it is due to Flysafair's terms and conditions. Which according to their website you automatically accept when making a booking. I tried to find this clause on their website but I couldn't. Juat the part where I automatically accept???

This cant be legal.