Speed Point Minimum purchase Amount

So this morning I went to BP garage Q section to put petrol while there I decided to go and buy something to eat for breakfast. To my surprise the cashier told me to add on what I was buying because it was below R20. I told her I do not need anything else she told me to pay cash, I told her I do not have any cash on me she said then she cannot help me as the minimum fee for card purchases is R20. I left without buying because although I needed what I wanted to buy but I did not have any cash on me. I was surprise that there is even a minimum payment for speed point purchases I mean why are the other shops not doing the same. I wonder if they are even allowed to do that.