Again, that would be a response to THEIR error.
What are they allowed to do when THEY make errors?
But it's not a mistake because a lot of businesses have models where they overbook, including the medical industry. Most times there is no problems, but sometimes you'll have to sit two hours in a doctor's office waiting for your appointment.
Airlines overbook because they have cancellations by passengers, missed connections, and they are okay with offering some rewards to passengers who are willing to be placed in a different queue. Passengers benefit in all of those categories.
So the model is: 30 minutes prior to departure they start boarding the flight but knowing they are or might be overbooked they offer people vouchers but keep boarding. At 10 mins till they close the doors. Under that model the group of people who arrive at the airport, whether it's from a connecting flight or a driving to the airport have an extra 20 minutes to catch the flight
Under the don't board policy, at 30 minutes prior to departure, the airlines have to make the decision on who not to board to avoid this situation. Who gets cut, first people arriving late at the counter, (so more people missing initial flights) followed by connecting flights (more missed flights) and they stop offering vouchers. All of these hurt consumers.