Put another way: banning oversell would mean that airlines routinely travel with more empty seats. This inefficiency would show through in higher ticket prices.
> In fact, it would be considered fraud.
Fraud involves deception. This doesn't apply in a market environment where it is widely known that they oversell.
The downside is that a free market isn't very good at adjusting to unlikely individual events.
I favour the legislative model in which airlines are permitted to oversell but there is statutory compensation due to anyone who is bumped. This provides the counterbalance to airlines taking it too far in risk to passengers.