Silly R9.90 Beverage Promo

Mcdonalds is running a promo where I can buy a beverage for R9.90 e.g. Standard Cappuccino. But when you buy a medium meal snd choose to swap the cooldrink for a standard cappuccino you get charged R21.00 instead of the R9.90 I am failing to see how this makes sense. When you question such things. You get bad attitude from the cashier.