People tend to use the Web site GoFundMe to set up fundraising campaigns for deeply personal causes—to treat a lame pet, for example, or to cover a niece’s medical bills. Recently, an unusual request turned up on the site. Aktarer Zaman, a twenty-two-year-old from Brooklyn, wanted to raise ten thousand dollars to fight a lawsuit that was brought by United Airlines and Orbitz against him and a Web site he had started to help people find cheap airplane tickets. “Skiplagged’s sole purpose has always been to help you become savvy travelers,” Zaman wrote on his fundraising page. “We have been doing that by exposing pricing inefficiencies for air travel, among other things.”

In the summer of 2013, after graduating with a major in computer science from Rensselaer Polytechnic Institute, in Troy, New York, Zaman was searching for a flight from New York City to Seattle and found that the best deal, for less than two hundred dollars, included a layover in San Francisco. On a whim, he searched for flights from New York to San Francisco and discovered that the New York-to-San Francisco leg of his flight was priced, if it were purchased as a single direct flight, at nearly three hundred dollars. This struck him, he told me, as strange and inefficient: Why wouldn't someone simply purchase the two-leg flight, if it were cheaper, and then just not fly on the second leg? He learned after some research that this practice is well documented and has a name: hidden-city ticketing.

Zaman set out to build a Web site and app, which he called Skiplagged, to let people search for hidden-city flight opportunities and then point them to third parties such as Orbitz and some airline Web sites, where they could buy the tickets. (Zaman wouldn’t tell me how this worked or how the site derived its revenues, calling those details “trade secrets”; he said, though, that none of his revenues have come directly from United or Orbitz.)

The problem, for Zaman, is that when passengers book plane tickets, they agree to what’s known as a contract of carriage—a document, dense with legalese, that can be found on an airline’s Web site. United’s contract prohibits hidden-city ticketing. (Most other big airlines have similar rules.) Also, many airlines have deals with travel agencies, including Orbitz, that bar them from facilitating the practice. Zaman seems to have been at least somewhat aware of this. He told me that he’d learned more about hidden-city ticketing from an article that Nate Silver wrote for the New York Times Magazine, in 2011. In it, Silver noted that airlines don’t allow the practice, but wrote that you weren't likely to get caught for trying it—and offered some tips on how to do it. According to the lawsuit, United last year asked Zaman to stop showing United’s hidden-city opportunities, and Orbitz demanded that he stop sending people to its site.

In November, representatives of the companies showed up at Zaman’s mother’s house, in Ozone Park, Queens, to try to serve Zaman with court papers. Zaman didn’t live there—he had his own place in Manhattan—and ended up learning of the lawsuit from a reporter. He wasn’t surprised. “It would be foolish for me to not expect that something like this would eventually come,” he said. Because he had registered Skiplagged as a business—an L.L.C.—only after learning of the lawsuit, Zaman could be held personally liable for any damages. After he realized how much his defense, or a legal settlement, might cost, he started the GoFundMe campaign.

United and Orbitz argue that Zaman, simply by operating Skiplagged, illegally “interfered” with United’s contract with customers and with Orbitz’s contracts with several airlines, by inducing breaches of those contracts. Zaman declined to comment in detail on his interactions with the companies, or about what he knew about their contracts, but he did tell me that he thinks that Skiplagged’s service is legal and that the plaintiffs have misrepresented his actions. He describes his site as an information service—albeit an advanced and specialized one—that publishes publicly available information and links to outside Web sites where people might make use of it. “Skiplagged doesn’t even touch United or Orbitz’s services,” he said. He argued that he is no more responsible for keeping track of the fine print on the sites that Skiplagged links to, and gets information from, than Google is for the sites it lists.

The holiday season, it turns out, is an excellent time to seek sympathy in a battle against air-travel companies. Zaman’s fundraising campaign has attracted national media attention, much of it sympathetic to his cause. “United and Orbitz sue Skiplagged, a service you should totally use,” read a headline on the Web site Boing Boing. In early December, Zaman answered questions on Reddit’s Ask Me Anything, or A.M.A., forum, and the thread became the eighth most popular A.M.A. of all time. Since then, Zaman has raised more than fifty thousand dollars and boosted his target to sixty thousand. (He said that he plans to give any excess funds to charity.) One contributor gave six hundred and sixty-six dollars and wrote, “Send them to hell, please.”

The strong reactions may stem from the notoriously complex and opaque nature of airfare pricing. Ticket costs can fluctuate wildly based on when they’re booked, the planned travel date, which cities are involved, how competitive a given route is, and how much passengers are willing to spend. The hidden-city loophole seems especially confounding, though. Given that a longer flight with a layover consumes more fuel, requires more hours of airline employees’ time, and so on, shouldn’t that flight cost more? One person wrote on Reddit during Zaman’s Q. & A. that the scenario resembled a department-store sale: “You could buy a Kit-Kat and a Snickers for less than the price of the Kit-Kat, but you only want a Kit-Kat. Is it unethical to buy the bundle and throw the Snickers out?”

The answer, on the face of it, seems like an obvious no. If a passenger has paid for both legs of a flight, isn’t it up to him to decide whether to travel on the second leg? But from United’s perspective, the situation is more complicated than it may seem. Christen David, a United spokeswoman, said in an e-mail that hidden-city ticketing is “tantamount to switching price tags to obtain a lower price on goods sold at department stores.” Her point is that the airline loses revenue when people use hidden-city ticketing in spite of the prohibition—making the act a form of theft. (Orbitz, for its part, said in a statement, “We have an obligation to uphold airline fare rules and also protect consumers from making purchases that could be invalidated.”)

Anyone who has suffered air-travel indignities while flying United might be disinclined to care whether the airline is hurt by the practice. But there may be some logic to United’s argument. What’s more, the airlines’ prohibition on hidden-city ticketing could ultimately be better for consumers.

Until the nineteen-seventies, airlines needed to seek permission from the federal government to operate along each route they sought to serve; the government also set fares that guaranteed certain returns to airlines. Consumers didn’t like the regime, which was seen as keeping airfares artificially high. So, in 1978, President Jimmy Carter signed into law the Airline Deregulation Act, which made it legal for airlines to set their own routes and rates. Suddenly, airlines could operate wherever they wanted, more or less, and offer tickets at whatever prices they chose; by the eighties, the big airlines’ business models had evolved to take advantage of that. They started using a hub-and-spoke model that allowed them to gather passengers from far-flung cities into a small number of hubs, and then redistribute them from those hubs; this let the airlines transport the same number of customers with fewer flights. As proponents of deregulation had predicted, airfares went down over time, partly because the hub-and-spoke approach was so efficient. But the model has led to new problems. Flights are far more interdependent, which means that one cancelled flight can have a domino effect on many others. Hub airports are congested at certain times and under-used at others. And, of course, there is the hidden-city problem.