Unjust Laws

Today I went to buy a KFC meal at Ushaka Marine World in Durban at around 17:30. I was served by cashier named Precious. I asked for 2pieces at that instant changed my mind and asked her make it 3. She said she cant change it and is not allowed to void. I asked for her supervisor Thandiwe Madlala who also cinfirmed that its againsts their policy to change I have to take what I initially requested or the cashier will have to pay for it as per their policy.

How is that possible how do you make cashiers pay for a void. The transaction was not even completed. Even so why can it not be returned so as a customer I should take what I no longer want so as to save the cashier from being charged.

Besides does it mean at KFC I cant change my mind whats written is written. Why really?