Not allowing me to bring my own drinks.

Nandos chargers R30-00 for a 2L drink,so after placing an order for my food I decided to buy my drink from a Spar nearby.When I returned to the restaurant,I went to our table and sat down,by then one of the waitresses already reported us to the manager,the manager comes to the table and tells me I'm not allowed to bring my own drinks because it's not a \product"of Nandos.By then we had already received our food and began eating.So I demanded that he take back my food and give me a full refund because I wasn't allowed to enjoy my food"