Bryan,
Because it is what is considered a limited public forum. There are certain regulations for use (just like posting policy of this board, although this is a private forum). Merely asking for "permission" or having to obtain a license prior to conducting constitutionally protected activities is not the defining element of whether something is a public forum or not.
Even public forums which we immediately think of as being "free to use" without a prior license such as a park or public street are subject to certain limitations. To host a rally in a park or to conduct a street march usually requires a license or permission from the government.
There has been much case-law on the issue of what is what is not a public forum and what is and what is not an unconstitutional restriction on the use of such a public forum.
Airports have traditionally been recognized as such public forums making them the traditional grounds of Hari Krishnas and those deaf people that come around leaving the card and thingy for you to buy from them. That JWs would use them too is neither new nor unusual, though because of the advance permission requirement, most do not set up a table - only because they are too lazy to go and get permission.
Actually, in the larger cities, Airports are these days pretty useless for pamphleteering or solicitation because of the security procedures and the fact that most of the passengers waiting to board or coming from planes are isolated from the rest of the people. (and the people at the points prior to the security checkpoints such as the lobby/ticketing area are too busy)
-Eduardo