Someone signs up with as many banner exchange programs as possible. They create a page on a free web server like FortuneCity or GeoCities and place all the banners on this page. They use every dirty trick in the book to get people to visit the site. By using timely keywords like Columbine in the page, they can get thousands of hits in a day. Their account is credited and they use the free impressions to promote their commercial site.
Most exchange programs don t view the participating web pages to check quality.
They seem much more concerned with unique page views. This is where many of your banner impressions may go if you use a free service. These services usually offer commercial banner ad space at a reasonable price, if you don't wish to participate in their exchange program. Mind what you re paying for, though. Some programs provide a list of participating sites.