First of all, I would be interested in seeing how many people, if any, have become witnesses because of someone sitting at a table at the airport. I would guess none.
I spend way too much time at airports, and I see them all the time. I have seen them a couple of times at a mall as well.
What bugs me the most, is these people are sitting there, usually doing absolutely nothing except talking to each other, and yet they sit there for several hours and get to count their time. I hardly compare going door to door with sitting behind a table where no one wants to talk to you anyways. Lazy, if you ask me.