Why does MTN not honour warranties?

On the 3rd Feb 2018 I received my Blackberry STV100-4, which I ordered online through MTN adding it onto my contract which I have had with MTN since 2008. Around the 20th June I noticed a small black dot on the screen of my phone, which grew progressively bigger with time. Then a purple vertical stripe appeared a week later which ran from the blob down to the bottom edge of the phone. 2 weeks later I took the phone to the Tokai MTN Store to hand it in for repairs...after all, the phone is only 4 and 1/2 months old. I dealt with a lady called Portia, who asked my for my invoice. I told her I had received the phone by courier and did not have an invoice as the phone was done through my MTN contract. She told me she couldn't help me without the invoice. I persevered and asked her why she could not check on her system and see that the phone was clearly showing on my contract. Still not interested. I asked her if I could then get a copy of the invoice from somebody. She said I should phone whoever we ordered the phone from. I asked her for the number for "whoever". She opened one of their magazines and pointed at a number and pushed it my way. I called the number and Alisha Pillay from MTN emailed a copy of the delivery POD. I then forwarded this to Portia's email address. She then proceeded to fill out the necessary docs and took the phone from me to be dispatched for repair. On the 23rd July she called me to say the phone was ready for collection. When I got there they said they were very sorry but the report back from the MTN technician stated that the phone would cost more than 75 % of the value of the phone to repair and it was therefore uneconomical to repair it. The report (Job Card Number OSTOK ********** ) states: "BEYOND ECONOMICAL REPAIR (BER)...NOTE: A BER determination is made when the cost of labour and spare parts exceeds 75 % of the current replacement value of a new device and when there is no economically viable repair solution or alternative available. It is recommended that the device be s****ped and disposed in an environmentally friendly manner." So they said so sorry you have no insurance you must just live with your black dot and stripe. When I asked what caused it, they said they had never seen anything like that before. The next day I called the MTN help line to find out my options. I asked if there was a warranty on the phone and she (another Portia) said yes. The reference no for the call is ********** 12. I told her that since the phone was still under warranty and they weren't prepared to fix it, they must then replace it. She became irritated and told me it would not happen because the default must be my fault. I assured her I had done nothing to the phone to warrant such a defect and that it was a manufacturing fault. She said my only other option was to hand it in for repairs again for another opinion and that I would have to pay a service fee as well, if I took this route. My question is: why does MTN not honour warranties?