Why no radio switch might be answered with the owner's use pattern they were designing for with these?
Disassembled awaiting sale from the store to the owner, then assembled in the hands of the owner who uses it regularly, was likely the way Segway thought it would be use.
A radio switch would just be another failure point and potential cause for accidents. Furthermore, a switch would hurt the design aesthetics. So advanced they don't have an off button
The minipro has a button but I bet a key fob simply couldn't be implemented in the build budget.