Speaking by my own words and what I know of these banners... they don't know EVERYTHING the banners show. Sometimes they just sign up and get paid to put the code for a banner on the forums, and the company chooses which will be shown.
Also, I see a lot of 'Get Free CD stuff'. One could also argue someone could fall into that trap too. Say I didn't like them, wy would they decide to take them down because of me? I'm not worth it to e-mail the ad company and ask them to please take off the free cd ads. If you don't like them, don't click them. I'm sure no one else really does anyways (except me... you know, since I'm a big gambler... bingo and that sort... high stakes sorta thing)