well those medical chair are not supposed to be on the road either. The bicycle has a special place in the law. The automobile has a place in the law, as does the motorcycle and more recently the moped. Each has a definition that makes it a motor vehicle even the unmotored bicycle.
However a wheelchair, and electric scooter, a medical scooter, a riding lawn mower they are all NOT motor vehicle and are not legal to have on the road. Cops don't really pay much attention, but they could. We have people in this neighborhood who ride their lawn mower to the grocery store, I think, but they are breaking the law.
There is no medical exemption for a medical scooter, lawnmower, child's scooter, or wheel chair they are not supposed to be on the roads period. That said most of the people who use them, just ignore the law and nobody bothers them because it's too much trouble, unless they have an accident then they are in trouble.
I try to obey the laws even the ones that don't get enforced. When there is no respect for the small laws it just encourages people to disregard any law they don't like.
I think if I built the three wheel e scooter, I could call it an e moped scooter and get away with it. I don't think a moped has to have pedals anymore. However there are new regulations coming soon to require licensing and registration of them I think. State needs the money and once there is a high profile accident with a moped at fault with no insurance, and none do, then they will be able to pass anything they want.