Strangely the answer is: most of them.
Weird as it seems major airports in Oslo, Stockholm, Hamburg, Lisbon, etc... want you to pay for the few megabytes of wifi connectivity you're able to use between flight connections. Others provide clumsy web kiosks to be fed with Euro coins. This is, or should be, a major embarrassment for any developed country as it represents the kind of impolite reception one does not expect from any other service. Speaking for mine: I feel embarrassed. But the problem is global, and deserves a bit of reflection.
For example, what would we think if airports started charging 1 Eur for the usage of luggage carts? Not that expensive but certainly very rude. What if we had to fill a form to open a water tap? Would be surely very inconvenient. Neither of these two situations would be acceptable, by European standards, even if being potentially profitable. That's because we accept certain implicit rules on the limits of what can be capitalized per unit of consumption. The costs of water and power are part of the average running costs of an airport, as they should be. So is the cost of gas, used for heating. But still, the water pressure, voltage and temperature must be kept at the right levels.
Why not the Internet then? Internet access is nowadays as standard as electrical power or running water. That's as everyone should see it. That's what the current generation of consumers expect. Why shouldn't we be able to answer a couple of emails or read some online news while waiting for a flight? Sounds a little harsh not to allow wifi access inside a large and professionally managed structure such as an airport, given the low cost of maintaining such a service. This is not only a matter of price, but also a mixture of convenience and politeness. No passenger wants to spend 10 of his 30 minutes of flight transfer figuring out how to use his credit card for spending a couple of Euros on yet another silly access point service.
So in the end I guess few people end up using these services. Few Euros are spent, few Euros are earned, some bits of work are delayed and a fast growing number of IT capable travelers gets deeply annoyed.
The situation, as it is, doesn't seem interesting to any of the parties: the passenger either doesn't use the service or must go through annoyances to pay for it, the airport doesn't look good in the picture and the service provider... well let's just say that no brand got more sticky in my brain than Telia Sonera in the last days. But that's the kind of brand awareness that wouldn't make their marketing department any happy.
Security may be an issue. Network managers running open networks can surely run into problems if their “anonymous users” are misbehaving on the Internet. However, authentication via First Name, Last Name and Ticket Number must be trivial to implement and enough to ensure that wifi enabled passengers can be identified before connecting.
