It's becoming increasingly obvious that airlines don't want anyone standing between them and you, their customer. They'd much rather that you buy airfares only from their own Web sites, rather than transacting through Orbitz, Kayak, or Travelocity. And to accomplish this, they're offering more and more promo code deals.

Look on many airline Web sites, and you'll now see a promo code box. Among the airlines that have added one: American, Southwest, United, Virgin America, JetBlue, WestJet, Air Canada, USA3000, AirTran, and Spirit.

Recently, Southwest offered 50 percent promo code discounts on all flights to/from both Salt Lake City and New Orleans. They also had a similar deal to/from Buffalo a few weeks ago.

JetBlue has frequent 10 percent off or $30 off promo code deals on their entire network, and they've even experimented with airfare auctions. Virgin has been known to offer 2 for 1 promo code deals. Other airlines have launched less alluring price cuts, in the $5 to $20 range, and American features DealFinder promo code discounts, typically ranging from 10 to 30 percent.

Why are airlines doing this? Because when consumers buy airfares on an airline web site directly, the airline doesn't need to pay a commission to the likes of Kayak and Cheaptickets. But there's perhaps an even more compelling reason: airline sites these days sell and market far more than airfares: they peddle hotels, rental cars, tours, frequent flyer memberships, frequent flyer credit cards, frequent flyer miles, SkyMall merchandise, and much more. You can't book a seat on many airline sites now without scrolling down through offers for adding a hotel or rental car. Airline Web sites are valuable properties and their owners want you to come visit directly and buy stuff beyond airfares, so they're providing incentives to do so in the form of promo code discounts.

How do consumers find out about promo code fares? One way is the sign up for the airlines' email newsletters and frequent flyer programs, since some promos are only offered to frequent flyer program members. Or they can visit Web sites like Airfarewatchdog.com, which lists every promo code airfare and deal on all airlines.