Because Only Terrorists Use 802.11
Anonymous free speech is one of the cornerstones of our democracy.
Sometimes people have things to say that the government doesn't want anyone to hear.
For them to be able to say it, they need to feel safe from reprisals.
People being able to post stuff on the net through an open wireless access point is one way that one can say something important without fear of official retribution.
For example, one could blow the whistle, anonymously, on a crime that has been committed by a government official. Reports of such official crime, for example presidential employees wiretapping the opposing political party, have already been enough to cause a regime change in the U.S. - most slashdotters are too young to remember but Dubya is old enough.
Most methods of internet access (such as all those AOL CDs) leave audit trails. They may not be able to prevent people from posting to the net, but they can track them down and either imprison or kill the posters afterwards, and to a large extent, the knowledge of that possibility is enough to prevent many people from speaking out.
So let me suggest that a good way to ensure a free and fair presidential election in 2004 would be to remove the password from your wireless access point.
Thank you for your attention.