If you don't know what a Segway is, it's a two-wheeled transport device that changes speed and direction depending on which way the user is leaning.
Is it completely unreasonable that Disneyland would prohibit Segways from the park? My answer is no. It would simply cause too much trouble to make every ride and attraction Segway-accessible. Remember that the park was built in 1955, without Segways in mind.
Personally, I fully encourage disabled persons to visit Disneyland. It is a very handicap-friendly environment, both in cast member service and in facilities. Many rides are designed to accommodate people with disabilities, including Space Mountain and It's A Small World. Maps are available in both Braille, and wheelchair-specific annotation. Most rides will escort handicapped guests straight to the front of the line. Even the stage shows have private sign-language showings for deaf or hearing-impaired guests.
But it seems that just because a new technology has reached the market, the park shouldn't have to fix all of these already-existent advantages. Until such a technology becomes available that it is an absolute necessity to the physically impaired, I think Disneyland should stay the way it is for now.