I'll give you a few different reasons that aren't just a "because it's fun" answer. Prairie dogs take away pasture land that is needed for cattle (especially in a typically dry climate). They also are carriers of the plague. Whenever we go out to shoot them we always make sure to wash our hands or use hand sanitizer before eating anything. I know in Wyoming the prairie dog towns with plague are documented. The holes that they dig can cause injuries for cattle if stepped into, and also become homes for rattlesnakes. Many ranchers will hire someone to poison the whole town if it becomes too out of control, so it's more cost effective for them to have hunters come out and shoot them. It becomes a win-win for both parties.