The operators of the fairgrounds want the revenue from RVer's when the grounds are not otherwise in use and generating income but still need to be maintained and policed. We usually find the rates at these types of facilities to be half that of a private RV park.
Towns also have RV overnight facilities, some with full hookups and others that are limited but in our experience these are also very quiet. Some are in city parks and others at city facilities like a water treatment plant. Not a good choice on the weekends but during the week they are worth considering.