Sarcastic and derogatory comments to customers

Had the most unusual experience today that has left me with a horrible taste in my mouth. Not too sure what to do or say, so just want some advice? It was my niece’s first birthday, and we went to the Mondeor Eco Restaurant in Somerset West for lunch. We have supported them for a long time and have always loved going there. Today was no different – we had delicious pizzas, lots of coffees and drinks, and had a wonderful time celebrating a little life – right up until the bill came. All the bills were settled, yet the owners/managers were apparently upset because the guests who attended did not spend enough money. As a result of this, the managers were so horrible to my mother in law, a 60 year old granny with the sweetest heart, and the manager pointed his crutches at her and accuses of her lying to him. When I asked him what the matter was, his wife absolutely belittled me, asked me how old I was, and asked me (sarcastically) if I could read, and told me that “folk like us weren’t welcome there”. This absolutely confused me? We have never had a horrible experience before at Mondeor, and had not caused any problems throughout the day. I told them that I was very shocked at the way they treated us, and that they had offended my mother in law and that they owed her an apology. The owner stood up and aggressively said that she owed him an apology. When I asked what for, he was unable to tell me. I left the place in absolute tears. The owner’s wife slammed the door behind us and said “good riddance”. I’m am just shocked that in this day and age you can treat customers like this. I told them that I was not impressed and that I would let others know how we were treated, and the owner’s wife threatened to send me a lawyer’s letter. I have no idea what to do or say, and have no idea why they treated us like this. My entire family left there hurt and upset. What a way to end such a special day :(

*update: We called the restaurant after we left to get an understanding why they were so rude to us. They said that we tried to “smuggle food in”. They were referring to the cake and cupcakes that we brought in with us, which we had called the day before to prearrange. They had agreed, as long as we paid cakage fees for the cakes, which we happily did. They did not apologize on the phone, but instead were rude to us again.