Overbooking on flieght

I am a very very dissatisfied customer today. My brother was booked on a flight from CT back home to Hoedspruit today. Upon check in he was informed that they overbooked with 6 people and he cannot fly today. They can maybe help him tomorrow but most probably only Wednesday!!!! No apologies, no help or vouchers for accommodation - just oops we overbooked. How on earth can this be fair practice? The ticket is paid for, he has to be back at work on Monday and you just shrug your shoulders??? Surely your booking system knows how many seats are on the plane? How dan you overbook?