Greed is what fuels the world. It is the unending desire for material things and non-material things alike. In a capitalist society, corporations are allowed almost free reign over how to make money, and their greed makes them stoop low in order to get it. There is no shortage of scumbags in the world who will do anything to make an extra buck.
For airlines, they will fill their plane to the brim in order to make as much money as possible. Sometimes, they do this without seeing if they require any extra space. If it turns out they require staff to board that flight, then they are allowed to kick passengers off the plane with some compensation.