United Airlines said a man wouldn't give up his spot on a flight. According to witnesses, he was pulled screaming from his seat by security and back to the terminal at Chicago's O'Hare International Airport. (The Washington Post)

Calls to boycott United Airlines are growing as outrage over the company's violent “re-accommodation” of an elderly doctor enters its second day.

While the incident has tapped into a deep vein of customer frustration with the practices of United and other airlines, many passengers promising a boycott may find flying without United to be harder than they're expecting — and in some areas, nearly impossible.

Thanks in part to a rash of airline mergers and consolidations in recent years, major airports are increasingly becoming one- or two-carrier affairs. Today, United commands over 50 percent of the market share in some places where it served fewer than five percent of air travelers ten years ago.

United could be the cheapest option serving a major air route. Or, it could be the only option. And that makes a boycott complicated.

Here are the major airports where United accounts for more than 10 percent of total passenger traffic.

United tops 50 percent market share in Houston and Newark. At other major hubs, including Dulles, San Francisco, Denver and O'Hare, anywhere from 28 to 43 percent of passengers are flying United flights.

The airline's 2010 merger with Continental has allowed it to greatly increase its market share in certain regions. For instance, at Houston and Newark United's share of passenger traffic exploded from less than 5 percent in 2006 to over 50 percent last year. That's primarily due to United's takeover of Continental's business.

The Department of Justice recently put the kibosh on United's plans to expand even further at Newark. According to the DOJ's complaint, “the enhancement of United’s dominant position would subject air-travel passengers at Newark — who already pay some of the highest fares in the nation — to higher fares and fewer choices.”

Still, consolidation is a fact of life, meaning that many American airports are dominated by just one or two carriers. All this concentration means that boycotting an airline is a potentially expensive proposition, particularly if your home airport is one of that airline's major hubs.

If I wanted to boycott say, McDonald's, for instance, I could simply take my business to Burger King instead. The two products cost the same and, let's be honest, more or less taste the same too — switching from one to the other does little to disrupt my pocketbook or my daily routine.

But competing airline flights between the same airports can differ radically in terms of price — and not much else. Aggregators like Google and Expedia have made the ticket price the primary factor distinguishing between competing airline services. At the price points most consumers are willing to pay, the flight experience is essentially identical across the major air carriers: cramped quarters, headaches, delays and a litany of TSA indignities before you board.

If I want to fly nonstop between Dulles and San Francisco (two United hubs), for instance, my options this weekend include a United flight for $632, and a Virgin America flight that costs $115 more. Virgin may offer better service — but is it $115 better?

I could be outraged enough by recent United controversies that I'll spring for the pricier Virgin America flight. But in all likelihood I'll opt for the cheaper United flight, pocket the extra hundred bucks and hope for the best.

For an airline boycott to work, in other words, customers would have to be really mad — mad enough to actually make a financial sacrifice. Judging by the ongoing reaction to the latest United incident, they very well might be.