British Airways

I bought a business class ticket for a flight on the 26th March 2021. The ticket price actually increased the day I booked. So now the new thing is, there is 3 economy seats in the business class section.. and generally the middle seat is empty . Not to be sold as this is the business class section of the plane. Guess what.. British Ariways sold the middle seat.. I think that is unfair... I since I paid for a business class seat... I paid to have space .. I could have bought an economy seat for 1/3 the price... I would like British Airways to tell me why.. why are they cheating their customers...