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For hotel guests, wi-fi is no longer a luxury like infinity pools, designer toiletries, or complimentary dry cleaning. Business travelers increasingly consider it a necessity, just as important as bottled water. A Hotels.com survey found that hotel guests would overwhelmingly choose free wi-fi over any other in-room amenity.

But not all hotels have caught on. Only 64 percent of hotels worldwide now offer wi-fi for free, according to the travel website HotelChatter’s 2013 wi-fi report.

And an expensive room in a fancy hotel won’t assure you the perk. In fact, some international chains are offering free wi-fi in their cheaper brands, but not their pricier digs. Hilton’s lower-end Garden Inn, as well as its extended stay hotels, offer free wi-fi to all guests; as does Starwood’s youth-focused Aloft brand, and the business-travel oriented Courtyard by Marriott. Ace Hotels, Quality Inn, and La Quinta offer it for free.

Those chains have realized wi-fi is a necessity for the business traveler or plugged-in Millennial served by these cheaper brands, explains Donna Quadri-Felitti, an associate professor at New York University’s Tisch Center for Hospitality, Tourism, and Sports Management. “It’s based on who their segment is,” she tells Quartz. “Courtyard is a brand for the business traveler. Aloft is a select service that focuses on the Millennial customer.”

But many of the higher-end brands—even within the same companies—are still holding out, and offering free wi-fi only to loyalty-club members. Intercontinental Hotels Group offers free wi-fi to all loyalty-club members. And club members at Hyatt, Hilton, and Starwood Hotels get to surf the web for free if they have gold or platinum status.

Marriott recently announced it is offering free basic wi-fi at the company’s eight luxury brand hotels, including the JW Marriott, Gaylord Hotels, and the Ritz-Carlton—but only for members of Marriott’s loyalty program, and users still have to pay around $5 to $7 for access to higher-speed internet that works for streaming video (gold and platinum loyalty members get it free). (Some have dismissed the move as a PR stunt following the company’s recent $60,000 fine for blocking guests’ wi-fi hotspots, but John Wolf, a spokesman for Marriott, tells Quartz that the move was simply a response to customer requests.)

It might seem perverse to refuse customers the free perk they most seek, but there’s actually a pretty simple explanation for it: Luxury hotels charge for wi-fi because they can. Those who are traveling for business can expense the $20 daily wi-fi fee, or if they’re paying for their own $500 room, $20 extra for wi-fi probably won’t break the bank. Budget travelers, on the other hand, are more price-sensitive. And unlike budget airlines that strip down the amenities to bare bones for a cheaper ticket prices, hotels in this sector have found that the promise of free wi-fi helps them compete with other hotels.

Of course, nothing is really “free”—the cost of the wi-fi is either bundled into the room price or offered à la carte. Wi-fi can be an expensive proposition for hotels. With more and more guests using the internet to share photos and stream video, the consumption of bandwidth adds up, said Quadri-Felitti. Installing the infrastructure for wi-fi for a hotel can also be expensive, said Jeremy Rock, founder of the RockIT Group, a company that provides technology services to the hospitality industry: A single cable could cost between $150 and $300, and each floor would need around 10 to 30 cables, he said.

Be that as it may, the tide seems to be turning against the idea of wi-fi as “optional.” Like budget hotels, ritzier establishments may well find it’s in their competitive interest to let the internet flow as freely as the complimentary shampoo.

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