Beyond the "people don't speed with a cop behind them" argument that jcb mentioned, The other reasons why I don't think the Segway tours are a fair comparison for the themeparks in general are 1. There is a LOT more room on the walkways in EPCOT's Future World than there are in many areas of the Magic Kingdom, Animal Kingdom, or MGM....so there is a bigger buffer around the Segways and non-segway public than in most other themepark environments, and 2. Think about the sea of people in some areas of the parks currently with stroller and
ECV's and the bumping that inevitably occurs. Do you really think it's possible for a device which is operated via the shifting of weight [steered/ accelerated/ braked] to operate safely in these situations? Even if you are the most highly trained and experienced Segway user (which there is no way for Disney to verify), you have no control over the other people around you during the post-wishes sea of people down Mainstreet and if they choose to bump or push you in their rush to get out of the park.
Ultimately, my only issue with the Segway in the park debate, is that I truly don't believe there is any way to operate a device which relies on balance and shifting of weight safely within the types of crowds I have witnessed at the parks. I've been bumped/shoved too many times while walking in some of those mass of people times (Park Exits, Fantasmic lines, Even exiting a show) to believe that it won't happen to a Segway rider.