Medicare will only pay an approved DME for scooters, and Segways are not available at DMEs. (A DME is a licensed medical equipment provider for anyone who does not know.)
Also, they aim to only give them to people who can't get around their home. They don't care about the disabled people being unable to go outside or do errands. You are only supposed to get a power scooter if you're unable to operate the manual wheelchair, and a power wheelchair only if you can't use the power scooter. Basically, they are trying to reduce costs by only giving the absolute minimum needed.
http://www.medicare.gov/Pubs/pdf/11046.pdf
I read that the DME sends someone to your home to decide which device to give you, after your doctor prescribes it as necessary.