"Refused on board": 6 million passengers stranded even though they have passports and boarding passes

The estimated figure is difficult to verify and appears enormous: each year, 6.6 million passengers are denied boarding, even if they have a valid ticket and passport, according to the results of a survey combined with data from the UK Civil Aviation Authority.
It could very well happen to you: you have your plane ticket in your pocket, yet you're denied boarding. It's the nightmare experienced every year by millions of air passengers who fall victim to the practice of "overbooking." This policy, whereby airlines sell more tickets than available seats, is gaining momentum.
The British study, combining surveys of a target audience and data from the Civil Aviation Authority, also allows us to estimate which travelers are most affected. Business travelers are at the top, with 38% saying they have been victims of overbooking, compared to 16% of vacationers. Those booking flights and hotels separately (19%) are also more exposed than those opting for package holidays (16%). And beware those traveling alone (21%) or with friends (24%)!

In the event of overbooking, the airline is legally required to take care of the affected passengers by offering alternative flights and compensation. But not everyone is treated equally. The study shows that 16%, or approximately 1 million passengers per year, were not rebooked. And of these, 71% lost money due to missed transfers or bookings.
The airlines' trick? Relying on a certain number of no-shows at boarding (on average 5%) to maximize the profitability of each flight. This statistical calculation allows them to offer low prices but comes at the expense of passengers. EasyJet acknowledged this a few years ago in comments reported by UFC Que Choisir. "Nearly three million EasyJet customers did not show up for boarding. […] Also, to be able to keep our prices low for all our customers, we may be forced to sell more seats than available for a flight if we know that some passengers will most likely not show up. We base this estimate on studying the history of each flight and examining past performance."
Think you're covered in case of a mishap? Think again. Most travel insurance policies don't cover overbooking, stating that the airline is responsible. Only some offer specific "travel disruption" coverage as an option.
The practice is legal but increasingly criticized by consumer associations. Remember that the airline is then forced to compensate you with a fixed scale that takes into account the total delay upon arrival and the expected distance. For a short-haul flight, expect to pay €250 (€125 if the delay does not exceed two hours) and up to €600 for a long-haul flight outside the European Union.
L'Internaute