I'm posting this from Melbourne Airport, in the cattle-class lounge, near Gate 5. There is an open wireless network in the area, but the airport is all about making money, of course, so they ask $4 for 15 minutes' internet access (or $20 for two hours).
Why? Having installed the infrastructure, they have practically no costs. I'm fortunate enough to have an alternate internet connection (3G), but for people who don't, there's no option. The airport moves huge amounts of money through, why do they have to squeeze an extra few dollars out of what could otherwise be a rather convenient service?
Coffee shops around the world offer internet access for nothing. You don't even have to buy a coffee, although it'd be a bit impolite to regularly use the service without. Does it hurt their business to give without charging? Apparently not, or they'd stop. But of course, the airport has to make money anywhere it can, lest they miss out on some cent of profit that they might have gained.