refuse to assist

I have receive a S4 almost 1.5 years ago. I have registered for the free screen replacement now only to be told that I will be responsible for the cost of the replacement if I can not provide proof of purchase. I don't have the proof of purchase but all the phone serial numbers etc. Surely they can see if the phone are blacklisted or not. Why can I not be assisted? What if the phone was bought from OLX or Gumtree? Am I now being discriminated against because I do not have a till slip? The phone movements can surely be traced. I am failing to understand that it is merely policy not to assist if you can not provide proof of purchase. It is good to lure people with your free options, but when it is required, your policy stands in the way....one word PATHETIC