I think it is easy to see by Disney's own strict regulation and use of the Segways it already has.
Firstly, the Epcot tour is around World Showcase when it is not open to the public/ very light early crowds- minimising the safety risk. Plus every user is required to take a test to show they can work the segway, and the additional speed restrictions keep all dangers to a minimum. Disney, if they allowed segways, would not be allowed to ask a person why they needed to use a segway or even ask if they had used one before, leaving them open to further problems if an incident occured.
Secondly the Fort Wilderness tour is not done in crowded areas, and they are done with specialised segways. Disney is a very crowded and congested area, that often needs harsh breaking and swerving to avoid children and adults alike. Disney does not use its own segways in these scenarios.
I understand that Segway users would be in the minority, but it opens Disney up to so much more litigation later on, is it worth it for them? To potentially alienate a very small section of their theme park users for the safety of everyone?
Disney takes every practical measure when it comes to
ECV's, they are as friendly and as concerned with accessibility as any company I know. If you don't believe me, listen to Kevin, listen to the DisUnplugged, cause they will keep on preaching how Disney go out of their way to make people with disabilities lives just that little bit easier, whether its just due to the high intensity of the parks or a constant in their lives.