That forcible eviction of a United Airlines passenger the other day is destined to become a prime case study in every Public Relations 101 class of what not to do if you’re in the business of selling something to people who have a choice.

The ugly incident, like everything else in modern life, was captured by nearby cellphones. It’s gone viral so millions have been treated to the sight of what happens in United’s Friendly Skies when they want you out of your seat after saying it’s OK. We saw the bloodied customer and his belly button as he was dragged down the airplane aisle, screaming he had patients in Louisville. Is there a doctor no longer in the house?

Now, there’s an attempted boycott of United. You could say United’s stock has crash-landed, except it’s unclear if it’s hit bottom yet. Already $700 million has been wiped from its market value.

The airline’s CEO, Oscar Munoz, has apologized and announced a detailed investigation, which veteran air travelers file under “It should only be a short delay, folks.” The CEO also pointed out the passenger was belligerent. No kidding. Standy for media stories exploring the ousted passenger’s background, all of which is irrelevant to the scene.

Unless you fly on Southwest Airlines, air passenger resentments have been building for years, ever since deregulation allowed them to start cutting corners on things we had grown accustomed to. Like resentments over, oh, say, entitlement reforms.

The grumbling began in the late eighties with American Airlines realizing it could save $40,000 every single day simply by leaving an olive out of its airborne salads. Seriously.

Airlines have been cutting back everywhere ever since. Remember free bags? Knee room? Pillows?

Then comes Sunday. The ousted passenger, David Dao, and three others were picked randomly from everyone who had already boarded. You, you, you and you out of here! Three went peacefully.

See, that’s the deal: Once, you’re onboard, it’s Go Time. Too late for any airline to renege.

If you’re still in the waiting area, that’s something else. Then maybe we can negotiate something. United had offered $400 and more for volunteers to fly later. No one bit. How about $500? $600? Right now, after this global corporate shaming, two-grand would seem like a real deal to United execs.

Initial reports said United had overbooked the flight. They all do this and it’s really passengers’ fault. Because some of us — and you know who you are — decide not to show but fail to tell the airline. But in good faith, it’s saving your seat.

To protect them$elve$ by maxing revenue, airlines book too many passengers, counting on no-shows to balance it out. But sometimes no-shows don’t not show. Which means too many butts for too few uncomfortable seats.

Ah, but there’s been an official change now in United’s story. Munoz explains that technically Flight #3411 was sold out with paying customers just fine. Paying butts meet equal number of seats.

However, United’s crew assignment folks discovered late they were going to be four crew members short in Louisville the next day. They needed to get them there. Crew-assigning is a very complex business, matching the right number of experienced people with sufficient allowable flight hours left in the month and required rest times to the correct model plane in the proper city at the appointed time.

Munoz has apologized again, this time deeply: “No one should ever be mistreated this way.” And he just announced a complete study of company policies. He’s ordered reviews of “crew movements, our policies for incentivizing volunteers in these situations, how we handle oversold situations and an examination of how we partner with airport authorities and local law enforcement.”

Other than that, everything is good now and our flight should be leaving shortly.