When an airline goes all out to make sure that you pay for inconveniencing you

By their own admission, they over book a flight and cancel online check in so that it becomes a first come first serve at the airport. I booked my flight 3 weeks ago, I was at the airport 40mins before departure time. I was told the flight is full at uShaka International Airport. Worse they make me pay an extra fee to be on standby on the following day flight, I had to incur extra cost of accommodation and transportation for something that was done on purpose, by making sure that we can’t check in online.

I left my son alone at home because I thought I’ll be sleeping at home. Now he’s crying nonstop on the phone and there is nothing I can do.