You have to remember that they're filming for days in order to find enough "interesting" material for the TV. Shows where people board on time, there are empty seats and no one fights just don't get the ratings.
All airlines overbook. It just looks like Southwest does it more than the other carriers because they're the ones who were willing to let themselves be put on camera and have their business shown.
Overbooking is more common (ie done more) with the discount carriers because the tickets tend to be so cheap. Last year SWA had flights on sale from Los Angeles to Las Vegas for $19 before taxes. How many people do you think booked those flights and then just didn't show up? The airlines have no way of knowing who's going to be a no-show, and they happen all the time. So if there are more people interested in the flight than seats, they just sell more seats and hope that people will either cancel or just not show up.
I've been on many flights that were oversold. Actually I think I've personally witnessed it less in the last few years. I even had Air Canada call me once the day before because they flight was oversold and they wanted to bump me before I even got to the airport. The compensation they gave me was more than I paid for the ticket, so I was happy.